Subject. I will get new Steam games from Humble bundles/monthly later, and I want to know, if I'll still be able to import them to GOG library or if it's a one-time option.

You mean with Galaxy 2.0 or GOG Connect?

Games are not permanently available through GOG Connect, but only for a certain period, and only when GOG and the publisher(s) agree that it's a good idea. At the time of writing, there is one game available - AI War 2.

Connect your accounts and remember to frequently take a look at the linked page above. If you miss them, you can hope that they become available again, but there is no telling when (or really, if).

Galaxy 2.0 (from what I've read, I don't personally use Galaxy at all) simply allows you to launch non-Galaxy games (not add them to your GOG account), similar to how Steam currently allows you to add non-Steam games to your list and launch them from Steam.

I think what the OP is asking is if games added to their steam account in the future won't ever have a chance to be ported by Gog Connect if they link their accounts today, provided they appear on the Gog Connect game list at a later time.

Answering that: No problem at all. Once you enter the Connect page you can click to refresh your steam account and Gog will fetch the new list of games in your account. If one of them is in the list of games available to connect it won't matter if it was added to your steam account the day before or 10 years earlier.

Just be aware that if you own multiple steam accounts you can only link one to your gog account; if that is the case choose wisely.