Q:
How to Know item aviability on Gemstore?
There isn’t a way. New additions are announced on the GW2 homepage on day of release, but they don’t announce beforehand
Most things that disappear rarely come back but the Silver-Fed Salvage-o-Matic is one of the more frequent re-releases. I think it comes around once or twice a year. As for when it’ll come back, that’s completely random and up to the whims of the devs. It’s a subtle marketing gimmick to get you to at least log in (beyond daily log in rewards) every day to check the gem store or to check the website, or read the news when you boot up the program (uncheck the auto-play to do this) to see if it has come back to the store.
If you read these forums regularly, you can be pretty sure that someone will post about unusual items in the store. I know that I give a shout out on my Enjin wall to my guildies when I spot something time limited they might want.
Otherwise, do a quick check of the store Promotions tab when you log in just to make sure nothing special has shown up.