The Mandalorian season 2 saw Grogu apprentice himself to Luke Skywalker, and he learned an important lesson from the Jedi Master - even if he did return to Din Djarin's side in The Book of Boba Fett. Originally named only the Child (and, out of the universe, Baby Yoda), Grogu was a survivor of Order 66. Sought after by an Imperial Remnant sometime after Return of the Jedi, he eventually found his way to Luke Skywalker.
Grogu's training didn't last long, however, as revealed in The Book of Boba Fett. Luke Skywalker feared Grogu had developed too strong an attachment to Din Djarin and placed him in the uncomfortable position of choosing between his surrogate father figure and the way of the Jedi. As Luke had likely expected, Grogu chose to return to Din Djarin, reuniting the found-family unit. Many viewers were somewhat surprised at the abrupt narrative course correction, but Grogu's time had clearly not been wasted. He proved himself more powerful than ever, easily taming Boba Fett's rampaging rancor.
In the Disney Gallery: The Book of Boba Fett special, Lucasfilm's Dave Filoni explained Grogu had indeed come to understand the nature of the Force from his time at Luke Skywalker's Jedi Temple. When asked what Grogu learned from Luke, Filoni stated that "It's not about this power to destroy, to become an expert swordsman." He then explained that Grogu's "power to calm something down" and abate intense emotions was "what being a Jedi and the Force is all about." By controlling the rancor, Grogu came away with those skills after his time with Luke. Filoni's point is an excellent one, and it suggests that - in spite of Luke's concerns - Grogu had bound himself to the light side of the Force.
How Powerful Is Grogu After Training With Luke Skywalker?
This naturally raises the question of just how powerful Grogu became after training with Luke Skywalker. Filoni's comments suggest he aligned with the light side of the Force, and thus likely no longer possessed the Force Heal power he demonstrated in The Mandalorian season 1. There have been hints this power is associated with balance in the Force rather than with the light or dark side, but he clearly mastered other powers, including the ability to enter the minds of animals and soothe them. All Jedi can skim animal minds, but only a few could accomplish Grogu's feat with the rancor.
Frankly, it feels as though Luke Skywalker presented Grogu with a false choice. Luke appeared to be repeating Yoda's mistakes, pushing the doctrine of non-attachment too far, failing to remember it was love that redeemed Darth Vader. He either did not know or does not understand that in the past, it has proved possible to be both a Mandalorian and a Jedi. Given this is the case, it's quite possible Grogu will become a more powerful Jedi than Luke could ever anticipate after The Mandalorian season 2 and The Book of Boba Fett.