what if gp had cleanse, quicksilver sash, mercury treads, and remove scurry skill

wont he be impossible to catch with any champion?

Blitz can still pull him, Alistar can still knock him up or headbutt him, Janna still knock him back, Gragas say Hi with his body slam and big barrel ultimate =]

Master Yi, press R and still faster than Gankplank.

If CC doesn't work, faster walking does.

Any position modifiers (other than Urgot's ult, that I can think of) cannot be cleansed, QSS'd, or W'd out of. Knock ups and knock backs still work. Blitz, Janna, Ali, Poppy, Vayne, J4, Cho'Gath, Darius, Jayce, Gragas, Lee Sin, Lulu, Malphite and a host of other champs can still CC Gangplank. If you really wanted to be troll, try running that build on Olaf. With R at level 3, Olaf is literally unstoppable for 8 seconds.

Edit: Skarner's Impale is another position modifier (assuming he runs with you stuck in his ultimate) that can be removed.

He still can't do anything..

OLAF AND GHOST. His ult stops ALL cc, Blitz pulls, everything, Then just ghost and you're uncatchable.