On Steam, the game was updated in 2022, 2023 and is still receiving updates in 2024.
https://store.steampowered.com/news/app/549080?updates=true

On GOG, the latest version I see is 12.30.21.
I only found out after purchasing.

Any reason for this?

This is also my question. On Steam are many updates, here none....

I got with someone who has the steam version of the game and his version also says 6.00 in the top. So i'm guessing it is all up to date.

I'm not convinced.
From what little info I could gather, version 12.30.21 was released on 2022-01-25.
https://www.gogdb.org/product/1121561784#changelog

I know the dates don't make sense, but I got with two people who have the steam versions of tank warfare and they both show 6.00 in the top right corner. Either way when the game goes on sale on steam I'm picking it up.