well lmp and gmp don't work together, but yeah, I'm not sure how a "projectile return" works with a projectile that explodes/splits. Would be interesting to see. In that case, you'd always want to just fire behind the mob/group, let it pierce through, then let them return again. Also if you go with a pierce-based Tornado shot build, you don't want to use fork since projectiles either pierce OR fork, but can't do both. Something like Tornado+GMP+Chain and the projectile return would be interesting.
Unfortunately, in my literally hundreds of hours of playing PoE, I've never seen a GMP drop. Ever. I've seen every skill gem in the game, and have probably had a dozen 5L items drop for me, but never a GMP gem, in any league. Blech. GMP drops are rarer than exalt drops for me.