I need some info before I buy. I wanna play with mod compilation (HSK), but they likely stuck at some old versions of Rimworld. Can I install old versions like 1.0 or less?

I used Galaxy 1.2 to install the game, you can see available versions on the screenshot. Unless there're older offline installers somewhere on GOG, I think the answer to your question is no.

in GOG Galaxy you can configure your installation, at "Beta-Channels" you can select "version-1.0". Maybe that will help you.

So can you install like A16 and play mods like MarsX if you get RimWorld on gog ?
If so it is cool. and I think I will buy it.

MarsX https://ludeon.com/forums/index.php?topic=25926.0
https://www.youtube.com/watch?v=eemhXMiE1pI&list=PLpgZPIJ2ZCLckeD3nTBpXvX3bFTCuozIm&index=5

hmm just realized this
Ludeon have apparently disabled download from the steam workshop.
test http://steamworkshopdownloader.com/view/732754899

Doing the same for donst starve works
test http://steamworkshopdownloader.com/view/2008373416

Bummer Ludeon

After a research, I've found that Steam release have this vesions Alpha4, Alpha13, Alpha14, Alpha15, Alpha16, Alpha17, Beta18, Beta19, Version1.0 and above + public testing. Thanks for answers.

So, goodbye, gog...