Yes, they must make the source public if its distributed at all to others in any form (like mods to load in client).
Just be forewarned, editing any code that has ever even touched or been linked with into a single jar with code that descended from gt5u must be lgpl and public as well.
And if any other mod there ever had GPL code as well then that infects the whole project and everything must be GPL as well if they link in any form directly at all and thus must be made public and GPL as well. LGPL can become GPL, but GPL cannot become anything else besides AGPL, and AGPL cannot become anything else at all.