No legal issues there because the iPad version was based on the old Dreamcast version whose code is legally owned by WeGoInteractive. The GOG version on the other hand was based on the old PC. WeGo also owns the PC version's code but this version contained many bugs and doesn't work on new versions of Windows. WeGo didn't fix the bugs, didn't make it work on nowadays systems and in order to sell it here without putting work on it, they downloaded a WIP (Work in progress) Patch from a fan site and gave it to GOG for sale. Just like this quote from G-Doc frrom GOG Team pretty much explains:
Hello, all.
It's my unpleasant duty to inform you, that as of today Re-Volt is going to be temporarily removed from GOG.com catalog. The reason for taking that radical step is a problem regarding the ownership of some parts of code used in the build we were provided for distribution.
It was brought to our attention, that the version of the game uses content modded in by the Re-Volt community (namely programmers known as Huki and Jigebren), but the authors of the said modification were never asked for permission to use their work commercially, nor they were properly credited for it.