My hostel wifi has strict internet policy. So if I play offline and then connect back when I get to home, will the achievements unlock?

I'm not sure but I think I got "What was that" offline

As far as I know it only works sometimes depending on your luck. I have noticed this in many of my games. For example achievements of metal slug, sleeping dogs and etc. Required internet connection but slain back from hell, absolute drift did not require internet to update achievements and once I switch on the internet the achievements pop up in the gog galaxy game page. Also do note I play most of my games without internet connection. But don't worry a future update of gog galaxy will solve this problem for sure.

Edit: Also, in my experience, witcher 3 and other cdpr games like cyberpunk 2077 updates achievements on their own once you go online after playing the game session offline the whole time.
Hope that helps :-).