I am thinking about switching to Linux. I'm fine with the idea that the newest games won't work on my PC but I couldn't be without some of these older classics, would GoG BG1-2 (and IWD1-2) work on Linux, Ubuntu?

Yes they do, but you have to get familiar with running Windows applications on Linux. This is mostly done with Wine, see triock's link above.

There's another alternative for the Infinity Engine games (like Baldur's Gate, IWD etc.), namely GemRB. GemRB is an open source rebuild of the Infinity Engine to run those games natively, that is without simulating a Windows environment. Though GemRB can be a bit buggy sometimes as it is still work in progress.
Here's a tutorial on GOG:
http://www.gog.com/forum/baldurs_gate_series/gemrb_linux_tutorial
Here's the GemRB's Wiki:
http://www.gemrb.org/wiki/doku.php

If you're interested in whether other GOG games run on Linux or not, check this thread and search for a specific game:
http://www.gog.com/forum/general/gog_games_that_are_working_fine_with_linux_wine

Or check the Wine AppDB for games that work on Linux using Wine:
https://appdb.winehq.org/

Thank you!

That is exactly what I needed.

The four of them work flawlessly on my Debian, it should be the same on an Ubuntu (which is Debian-based).

if you want to run them natively and not through wine, take a look at GEMRB.

for me it was easier to configure that, than to make wine work.

(edit) silly me. I read the previous posts too fast, and didn't see this was already said.
sigh. If only I could delete my post....