F4 xx clone is the only safe clone cancel GM has. Ex dash fist and launch him if he tries it off any other string/ normal. If he burns a bar on clone, however, he's safe after any string. F4 xx ex clone is a frame trap. Not sure what Jax jip/ jik looks like, but many characters can simply jump over unsafe clones and start a combo/ mix up after unsafe clone strings (I'm able to do this in SZ mirrors).
I wouldn't ground pound too often. Better Sub Zero's are looking for ways to get in and corner you. Raw ground pound is just what he needs, and likely won't work more than once.
Once in the corner, no wake up Jax has is safe. GM can beat every corner armor option you have with an f12 ender, clone, d4 check. D4 will take armor and clone will freeze you after. Better players can fit in a b2, f4, or even a d2 for an extra, unscaled 14% and still freeze you back into the same set up. NJP is a popular option vs GM's corner set ups, but we know this the same as opponents do.
Respect clone in neutral. As a pressure character, know that SZ controls the pace this time. The worst thing Jax can do is walk backwards away from clone. Stay right next to it (be wary of throw into clone though). You can run up block the clone to shatter it, but it takes practice. And still this means you're going to take 1 strike/ throw mix up. The only exception is f33. It's -3 for Sub and is likely a bait so that he may ex frost bomb Jax's counter action. But after successfully blocking that or the f33 bait, pressure belongs to Jax. Do what you do best at this point.
SZ can low profile Jax projectiles despite them being mid with slides. Ex ice blast will eat Jax ex projo's as well. Any trade with clone toss is a bad one unless it's with armor.