Matches Gog Low
Last Discounted 7-Oct-2019

Does the game have a free play or sandbox mode? Or is just challenges?

Played it while it was on early access, it's a fun game.
I wasn't very good at creating things, but some managed to build cars that transform into flying mechs. :)

Oh God! It's outrageous! The abominable WinXP is mentioned in SysReq section! Haram! It's not supported anymore by nobody, and nobody uses it anymore, never and nowhere! Alarm! It's Russian hackers, sponsored by Chinese hackers, hacked GOG! Delete it immediately! Oh my poor eyes! My poor mind! My world is crumbling! I'm dying...

Nah, it just means the developers must still be using Unity 5. (Or possibly one of the earlier versions of Unity 2017, but that seems unlikely.)

last i played it, it did.

I'm surprised the base price is only $10, is it a relatively simple and short game?

Besiege: The Splintered Sea Announcement

"Dive into The Splintered Sea expansion, introducing water to Besiege for more machine-building conquests! Battle your way through a 10-level oceanic campaign, explore an all new sandbox, and harness 8 new blocks to unleash chaos on the high seas!"

The Splintered Sea (Feature Trailer)

"The Splintered Sea expansion is OUT NOW and introduces water physics to Besiege for more machine-building conquests! Battle your way through a 10-level oceanic campaign, explore an all new sandbox, and harness 8 new blocks to unleash chaos on the high seas!"

Maybe someone with access to the forums over on the other platform could poke Spiderling Studios about the ETA for the expansion to hit GoG?
Thanks in advance.

In the 2nd class thread someone linked an official post that that said it will stay Steam-only.

https://www.gog.com/forum/general/games_that_treat_gog_customers_as_second_class_citizens_v2/post4355