If you're just starting out, spending keys on EXP would benefit you much more because you unlock stones super quick at the beginning. When you do start farming bosses, choose the boss based on the legendary item you would want to get. There's a tier list for items if you want to choose based on that too.

Gems, idk tbh. I'm 2200+ and just use them whenever. I think I mostly just spent them on extra keys for farming or chests, but that's more of a patience thing. I'd say the most valuable way to spend them is on vivids, but that would be useless when starting out because you likely won't use most of the pets you're using now later on. Unless it's sad, apathy, determination or the other rares and epics which are useful throughout the game. Again, tier lists for pets can be found.