I personally disagree with the idea that gyms should teach type matchups. Type mechanics are a pretty basic thing that can be covered briefly in the start of the game (if it's even necessary-fangame players are more likely to be experienced with them), while gyms are a mechanic that lasts almost the entire game. Type matchups could be taught much more easily through a trainer's school with a variety of types, or an NPC that would explain them (like Cheren in BW2). They also don't really teach you the type's property as much as they teach how to deal with a type. When fighting Jasmine in Crystal, for example, I didn't learn a dang thing about all of Steel's resistances-I just learned that Fire-type moves were super effective, and used that the entire match. It's still a great lesson, but still not something hard to learn.
I'm slightly more fond of gyms with a "theme" of Pokemon (like Uranium's second, third, and 6-8th gyms), because it's a more interesting way to look at Pokemon (I can sort through my dex to see all the Pokemon of one type, but what about all the Pokemon associated with the sun and moon?). But nobody ever said that you can't do that with a monotype team, either-just look at Regina's gym in Ethereal Gates! Even though her type specialty of Normal is a hard concept to work into a gym design, rather than just focusing on the type, her gym is a massive barn that reflects the environment of the Pokemon inside! (and the barn rafters where you battle Regina look absolutely stunning-but what else would you expect from Ethereal gates?) And while a monotype team might be harder to make a challenging team with, it also has its perks-players are likely to bring a type super effective against them, so you can keep a couple moves for coverage! (especially useful for types like Fire and Dark, where all types super effective against them have a shared weakness) And the inevitable +1 for Whitney's Miltank-Rollout was so simple, but so stupidly hard, especially for those that had been cruising with their Cyndaquil/Quilava until then.
(Also-for all that is holy, please, please don't make your first gym "straight line, two trainers, gym leader", especially if they're a Normal/Rock type specialist. It's been done too many times!)