I recently purchased the witcher 3 expansions from eneba. i already have the game in my steam account. although expansion pack was for GOG and i didn't notice. i have linked my GOG account with Steam and i still cannot redeem the expansion. somebody please help

if i buy the witcher 3 game for GOG so that i can activate the expansion (sinse i have linked Steam with GOG) will i have my save from steam?

Enabe is key reseller site. Those are shady sites, often (re)selling keys which were purchased with stolen payment information or through other obscure sources. You might also get a regionally locked key eventhough the site says it should work in your region.
GOG.com has removed games from accounts when they were purchased with stolen payment information.
GOG games (with some exceptions) can only be bought here!

You can only redeem the expansions here, if you also own the base game here.
During a recent sale you could use GOG connect to redeem a GOG copy of the game if you have it on Steam. Unfortunately that was (to my knowledge( a temporary action.

Hello, I've bought the witcher 3 expansion pass on Instant Gaming but when i use the code on GOG.com it's the witcher 3 goty edition wich appears on my library GOG.COM. But if I play with the goty edition can I use my old game because I don't want my progress in the game (classic edition) to be lost on the goty edition ?

The save should work, unless the version you currently use is seriously out of date in which case you might have an issue. 1.3x shouldn't have an issue, not sure about older versions.
If you have TW3 and the expansions here, GOG also gives you the GOTY. It's the same, but in 1 installer.

IG is also a key reseller site, see my previous comment in this thread.