Please note: This is speculation. I don't necessarily think Jay is Zoom (actually, I'm afraid but still thinking that they'll go with the obvious guy, based on the comics), but it is possible.
To start with, Jay Garrick is NOT Zoom in the comics. (Another guy is, wink wink.)
OK, Jay Garrick is without speed, he has yellow lightning, not blue, and he just couldn't be Zoom! He's a good guy!
So was Harrison Wells (Eobard Thawne), right? Nope. It was a clever giuse.
But Jay wants to "help" Barry get more powerful! So does Zoom. The stronger Barry is, the more energy can be taken from him. (Earth-2) Harry correctly guesses Zoom is "fattening him up like a Christmas goose". Jay taught Barry about the Earth-2 metahumans and helped him beat them.
Jay has yellow lightning, as seen when he runs to face Geomancer. But it's never explained how the lightning's color is determined, so who says he couldn't change it? Maybe the speed at which a speedster runs determines the color of his streak. Or perhaps the Velocity-7 did something to change the color.
Jay knows the ins and outs of the speed cannon. And guess what, someone else does too. We see Zoom using it to enter our Earth. Do we ever see events involving Jay and Zoom simultaneously? (If you know of any, please inform me so I can check them out.)
It would make sense for Zoom to be from Earth-2. And how does Jay know about Zoom's plan to eliminate all other speedsters in the multiverse? Zoom probably would have killed him if he'd found out the plan.
Jay says the Velocity-6 is killing him. Did he take it to become more powerful, but when it failed, he had to try something else to become stronger? He could take the essence of the Speed Force from another speedster, say... Barry Allen?
Also, Jay was adamant about not taking V-6 ever again. Even when Harry got shot, he said no. Then Caitlin said he should help, and voila, he did. He saved the guy who was aiding Zoom in taking Barry's speed. Also, when he took out the bullet, he phased through Harry's chest like Zoom (and Thawne) did when he killed Cisco.
He could easily pose as a hero while being a villain. That's just plain classic. And since he doesn't wear a mask, who would suspect him of being bad? And if Jay and Zoom have been seen fighting, who's to say it's not just a ruse similar to what Eobard Thawne did?
There's a flashback scene in which Jay sort of "escapes" from Zoom. But nobody was there to witness it. And who says all the breaches are in Central City?
We'll see soon enough who Zoom is. I could see him being Jay, but I could just as easily see him being someone else. In the meantime, share your thoughts about this post.