I wonder why so many games are missing achievements on GOG. I'm not a developer but if it's as simple as adding a plugin to Unreal 4, and this game's engine is Unreal 4, what are you guys waiting for?
Sometimes you will have to write such a plugin (like the devs from Strangeland did) and sometimes an existing plugin is not working as intended. However it MIGHT be additional work. When it comes to WIRED they do NOT intend to invest any additional work into the game - since this is the reply I got from them today.
"Sorry for any confusion here – in this instance, GOG themselves implemented the game in its whole, this wasn’t a version the developer worked on. This version is simply a DRM free version of the game – no achievements or online, just pure combat racing, solo or couch co-op, all DLC bar the DeLorean. This is advertised and we do not have any plans to bring other features into this version. Again, sorry for any confusion here and we hope you still manage to enjoy GRIP on GOG."