Thankfully, evidence forshadows a genderlock. Virgil had every Eeveelution as a male accept the unevolved female Eevee. She will probably be a Sylveon later on. Also, in Pikachu and Eevee friends, it shows Meowth falling for a previously unknown pokemon, aka Sylveon. If it weren't genderlocked, I doubt they would have done these two things in the anime. Plus, look at Froslass and Gallade. It is very likely
oh man, i don't like this, not at all. to me the whole genderlock-thing really sucks and seems very pointless, except for the obvious ones like miltank and tauros. i'm still keeping my fingers crossed for my possible male sylveon!
that wouldn't make sense since eevee hasn't been able to evolve with a shiny stone before, so it's probably either a brand new evolving method or tangrowth etc. -style method (knowing a move and then leveling up) but we'll see!