Although they were always portrayed as the best of friends (it's right there in the name of the show!) it's been well-established by now that the Friends gang weren't actually all that nice. They were mean to each other, tended to lie a lot, made fun of each other (and everyone else), and were generally more than a little problematic as actual friends. But they weren't just mean to each other...
Throughout the show, the Friends crew are pretty terrible to all the other people in their lives, too. It might be family members, romantic partners, or colleagues, but they definitely don't treat other people the way they should... and one prime example is Gunther. The kind and shy manager of Central Perk, Gunther gets the short end of the stick often enough - especially considering these moments.
Rachel Gave Him No Respect As A Manager
By the end of the show, Rachel is working in her dream job in fashion, which is fantastic for her... but when she first moved to the big city and needed a job, she ended up serving coffee at Central Perk. She was also a truly terrible waitress, so much so that her friends even expected her to mess their orders up. She didn't really give her job any of the effort that she should have (because she hated it), but disliking the job still doesn't mean that she should have treated Gunther with so little respect when she was actually his employee. Anyone who wasn't secretly in love with her would have fired her long ago, but Rachel was so spoiled she just expected to get away with it all.
Chandler Called His Job Humiliating
Not behind his back, either. When Chandler was considering a career change, and wanted to get into advertising, he got offered an internship. A great way to break into a new industry, but like most of the gang, Chandler was a bit too entitled to actually want to do the grunt work. Talking about the training program (with a few jobs up for grabs at the end), he says "a grown man getting people coffee is a little humiliating"... right as Gunther, a grown man, comes on by to serve him the coffee he ordered. Ouch.
Chandler Assumed His Last Name Was 'Central Perk'
His 'humiliating' comment wasn't the only time that Chandler was a little less than kind to Gunther. At Ross's bachelor party, Joey and Chandler are arguing about who would be the better best man, and Chandler is annoyed... and announces that Gunther would be his best man. However, Gunther actually stands up for himself for once, sarcastically asking Chandler his last name - which Chandler would know, if they were actually close enough for a best man job. Not only does Chandler have no idea of the last name of someone he sees almost daily, but he is insulting enough to guess that it is 'Gunther Central Perk'.
Ross Routinely Left Him Out (Even When He Asked To Join In)
Gunther was in a weird halfway place for the gang. He worked with more than one of them, over the years, they saw him every day, and he often popped up at parties... but most of the time, the group totally ignored him, and treated him like any random barista. They even went out of their way to leave him out sometimes.
When Ross came into the coffee shop to celebrate with a bottle of champagne, he asks Gunther for six glasses. Fans' hearts broke when Gunther's face lit up and he asked if he was joining them... but no, Ross didn't realize that one of the core crew was missing, and rather than actually just invite Gunther to have a drink, he actually refused him.
Rachel Made Him Buy A Horrible Cat
All of the gang have been known to take advantage of Gunther's love of Rachel from time to time - but this may be the most egregious instance. Rachel, fairly randomly, decided to buy a hairless cat one day. She used to play with one as a child, and was apparently unaware that not all animals have the same personality - so when the cat wasn't as playful, she wanted to sell it on. Not only did she get Gunther to buy the cat from her, even though he didn't want it, but she actually charged him more than she paid, even though the original price was in the thousands.
Phoebe Kissed Him Without His Permission
In one episode, Phoebe was thrilled to get a cold, because she thought that it made her singing voice huskier and more sexy. Which is fine (if a little gross), but when she got better, she was miserable, and started actively trying to get sick again. Also fine, but gross... until she heard Gunther sneeze, grabbed him, and forcefully made out with him. Definitely unacceptable on all levels, and to make things worse, she then totally ignores him immediately afterwards.
They All Ignored Him (Except When It Suited Them)
Of course, Phoebe is far from the only one to ignore poor Gunther - they all do. Everyone seems wildly oblivious to the fact that he is completely in love with Rachel, even though it couldn't be more obvious (and they do use it to their advantage, because who cares about someone else's feelings, right?).
He often does fairly weird things because of this love, too - and yet, they totally ignore it, every time. In fact, they seem to ignore him all the time, unless they need something (including to make up numbers at a party), and yet lead him on to believe that he is actually a friend.
Joey Acted Like Taking A Job Was A Favor (After Not Paying His Bill)
One of the best examples of the gang taking advantage of him, of course, was when Joey used their connection to get a job at Central Perk when he needed money. This started off badly, too, as Joey actually had the gall to go in, order food and drink, and then just tell Gunther that he didn't have the money to pay - something that would get the cops called in some places! Then, when Gunther kindly offered him work to help out, Joey acted like he was far too good for the work, tried to keep it secret, and was generally awful at his job.
...And Then Rachel Yelled At Him When Joey Was Fired
Unsurprisingly, Joey was fired. Not only was he terrible at his job, but he asked to leave his shift for an audition - and when Gunther said no, because he already had an appointment and had to leave Joey to cover for him for an hour, Joey literally kicked all the customers out and closed up shop. Whether Gunther should have been able to leave or not, this is clearly a fire-able offense... but Rachel uses her pull with Gunther to go and yell at him (which is ridiculous), and of course, Gunther caves and gives Joey his job back.
None Of Them Ever Invite Him To Weddings
Over the course of the show, almost every member of the gang gets married. In some cases, they even get married more than once! However, despite seeing Gunther daily and inviting him to other parties (where they want the event to seem popular, but aren't paying per head!)... he's only at one. They will invite random distant relatives and near-strangers at times, but never poor Gunther. To make things worse, the one wedding Gunther does actually get invited to is Phoebe's - which is at Central Perk, thanks to a snowstorm. Had the original wedding plan happened, it's likely that Gunther wouldn't have been there either - and he probably knows it.