Although he's been around since the pilot, Spike is often neglected in the show proper. While a handful of episodes give him solid character development, and while he is an enjoyable and interesting character in his own right, there is less attention given to him than the main six and arguably even the Cutie Mark Crusaders. After a particular dry spell in season 5, where the only Spike episode was the rather uninteresting "Princess Spike," we get an episode that doesn't particularly develop him or any other main characters, but which provides a somewhat interesting glance at the culture and society of Equestria's dragons and possibly causes major changes to the status quo.
While helping Rarity search for gemstones, Spike spontaneously starts glowing and experiencing a horrible burning sensation, and is taken to Twilight to see if she knows anything about it. Celestia and Luna just happen to be there for a visit, and they quickly inform Rarity and Twilight that Spike is being summoned by the Dragon Lord. In order to make the burning go away, Rarity and Twilight accompany him to the Dragon Lands in order to answer the call, where they learn that Spike was summoned to compete to become the Dragon Lord's successor.
What follows is a modestly entertaining episode which isn't the funniest or most exciting the show's ever been, but is still a reasonably engaging episode which is mainly propped up by some fun interplay between Spike, Garble (the bully dragon from season 2's "Dragon Quest") and the Dragon Lord's daughter, Ember. Overhearing Garble's plans to take revenge on the ponies, Spike realizes that he might need to become the Dragon Lord in order to protect the ponies, and his quest to complete the required trials to claim the title is entertaining in its own right. Rarity and Twilight occasionally chime in with a quip or two, and their interactions with Spike are as charming as they have been in previous episodes, but the real star of the episode is the growing friendship between Ember and Spike.
As a character, Ember is more or less just an amalgamation of tropes, most prominently the "underestimated princess" trope. Her father, Dragon Lord Torch, doesn't want her to participate because he doesn't think she's capable, and honestly it's not the most interesting story element. A little more interesting is her reaction to the idea of friendship, something that dragons in My Little Pony are notoriously averse to. While the idea that dragons don't make friends isn't entirely plausible, it's significantly better here than it was in season 5's "The Lost Treasure of Griffonstone," because at least the Dragon Lands don't seem to be in a recession. Ember is significantly more clever than many other dragons, and when Spike acts to help her, she takes an interest in his actions and begins considering this "friendship" thing. This character arc concludes exactly as you'd expect, but seeing her come to accept Spike's friendship is fairly satisfying nonetheless.
As this is the first we've seen of dragons since season 2, it's nice to learn a little more about their social structure. Dragon society isn't particularly interesting in and of itself, but there's some nice deviation from how dragons are traditionally depicted. The way dragon society is focused around a single dragon lord, while not particularly novel as far as the brutish races in fantasy go, is kinda neat, and the idea that a dragon's body reacts intensely to a magical summons is a fairly original and interesting idea. More significantly, Ember's inevitable ascension to the throne potentially alters the dynamic between ponies and dragons, and although I doubt the show will make much of it, this might be the start of pony-dragon relations, if not an outright shift in dragon society.
Unfortunately, focusing this episode on a newly-introduced character weakens its foundations a little. Ember's character development is endearing, and the worldbuilding is mildly interesting, but aside from the predictable conclusion, there's not a whole lot exciting that happens here. Neither Spike nor the ponies he brought with him develop much, and the episode doesn't have a particularly strong emotional or thematic core. It's fun, sure, but it's not really the most fun the show's ever been, even if it's still far more watchable than the least interesting parts of season 5. But then, after the excitement and attempted complexity of the rest of the season so far, maybe a simpler episode is just what was needed. "On Your Marks" suffered from attempting to do too much, so an episode which has more focus and a distinct (though predictable) end point serves as a nice change of pace from the earlier episodes.
"Gauntlet of Fire" isn't as exciting as the previous few episodes, but it's fairly charming in its own right, introducing a likeable new character and incorporating some fairly interesting worldbuilding. It remains to be seen whether the changes to the status quo at the end will have any effect on the rest of the season, but they're satisfying in their own right. As a change in pace from the turbulence of the first four episodes of the season, they're a nice breather as we wait for future episodes which will hopefully provide the level of character development and story progression that the season has promised so far. It's a fun episode which continues season 6's trend of moving the show forward, and that's more than enough.