Depiction in the series
The gala is first mentioned in The Ticket Master. Spike reads that "the Grand Galloping Gala [is] to be held in the magnificent capital city of Canterlot, on the 21st day of" before skipping to the end of the letter. None of the Mane Six had ever attended before, and they each describe to Twilight Sparkle how they'd spend their time at the gala. Pinkie Pie believes it is the single biggest party in all of Equestria, filled with snacks, decorations, and party games. Rainbow Dash proclaims that the Wonderbolts perform every year and imagines herself performing with them. Fluttershy says that all the flowers in Canterlot's private garden are in bloom during the night of the event, with some of the most exotic avian wildlife in Equestria present. Rarity mentions that every year she designs dress attire for clients who have been invited to the Gala; the plot of the episode Suited For Success revolves around Rarity designing gowns for the Gala for her friends. She says she hopes to meet a prince there, believing they will fall in love and marry. Twilight thinks it will be an opportunity to meet with Princess Celestia and talk about her adventures in Ponyville. Applejack claims the Gala will be the perfect place to sell her apple-based homemade food so she can use the money to fix the roof on her barn, Big McIntosh's plow, and Granny Smith's hip. Finally, Spike claims that he wants "no part of this girly gala gunk", and pretends to be disdainful about the ticket that Celestia gives him.At the end of the season in The Best Night Ever, all of the six ponies' expectations are crushed: Pinkie Pie is let down by the lack of partying; Rainbow Dash, though she does get to meet the Wonderbolts, doesn't get to perform or spend much time with them; Fluttershy explores the private garden but fails to befriend the animals, causing her to snap and start terrorizing them; Rarity meets a prince, Prince Blueblood, but although he looks exactly like the one in her imagination, he is very snobbish and off-putting; Twilight meets the princess, but finds she is busy greeting all the gala goers; and Applejack sets up a food stand but only manages to sell to Soarin.
The Mane Six attempt to make it special in their own way, only to have everything literally fall apart and the party descend into total chaos, causing the Mane Six to run away. As they rue their "worst night ever" in Joe's shop, Princess Celestia arrives and states that she finds the gala to be "awful" every year, and that they had made the Gala livelier with their involvement.
In Sweet and Elite, Twilight Sparkle notes that "the Grand Galloping Gala [is] coming up". Earlier in the episode, Rarity mentions the gala.
The Grand Galloping Gala makes another appearance in Make New Friends but Keep Discord, with Twilight helping Celestia host the event to ensure she enjoys the festivities for once. Each of Twilight's friends is also invited and brings a plus one to the event: Fluttershy takes her new friend Tree Hugger; Applejack, Rarity, and Rainbow Dash take the Cutie Mark Crusaders; and Pinkie Pie invites her sister Maud Pie. Princess Celestia also invites Discord in the hopes that he might liven up the event. Having received his ticket late due to the mail pony Parcel Post getting lost, Discord feels snubbed for not being Fluttershy's plus one and invites another friend, the Smooze, which Twilight fears will ruin the event. The Smooze quickly begins devouring all the treasures and valuables he can find at the Gala, growing in size until he floods the entire floor with his slimy form. Tree Hugger eventually appeases the Smooze while Discord restores him to his original size, and the Gala continues as normal. To Twilight's shock, Celestia declares it to be "the most fun Gala in years" thanks to Discord.
Depiction in Equestria Girls
According to the iOS app Twilight Sparkle: Teacher for a Day, the Grand Galloping Gala originated when the unicorns celebrated the completion of the construction of Canterlot and invited the other ponies to a huge party, which was greatly enjoyed by all. The party is therefore organized once every year ever since then, and continues presently.
One of the stories in the French magazine is about the Gala, albeit does not follow the same continuity as the television series.
WeLoveFine's descriptions of their formerly available men's T-shirt and hoodie "Magnet Pony" mention the Gala. The "Magnet Pony", whom is actually a fan-made pony version of the X-Men character Magneto, is based on an inside joke by a group of bronies whom were inspired by an advert that randomly appeared on a notable live-stream of The Best Night Ever when it made its debut airing.
On the official My Little Pony Twitter and Facebook pages, the Gala apparently takes place on New Year's Eve.