Is there a rotation schedule in gem store?

I mean that bank store is 50%off for like.....2-3 months now....will it ever change to something else?

Not specifically, no, though there are daily sale offerings in March and August. If you go to the wiki page for the item you're interested in, there should be a Gem Store History section that will tell you all the times it has been on sale in the past, and you can get a rough idea of when it might be on sale again in the future.

I think they deliberately make the gem store unpredictable so players have an incentive to keep checking, and so it's harder to save up or stockpile gems ahead of an item you want appearing - making it more likely players will resort to using real money.

They do change items around regularly, but only a few things at a time. For example they recently removed a bunch of outfits which had been available for months and replaced them with ones which had been unavailable for ages. If you're waiting for a specific item to be available, or to be discounted, it could be a long wait. Which is annoying but apparently it's an effective marketing technique.