I'm going to have to disagree with Dekar. Don't shield his turtle. An experienced G&W will position himself so that the turtle will land the hits on you, but you won't be able to grab through them during the attack. (If they are low enough, you can) This means that your shield will take a beating over and over. Eventually the turtle will eat right throuh and hit you, I've even seen the shield break a couple times. Just keep popping out them naners. I've found that glide toss backwards + throw down is effective, since the trajectory G&W uses w/ turtle places him almost on top of the banana.
As for the edge guard, just keep practicing and get a feel for when g&w is vulnerable after the down smash. When the hammers come down, he won't be able to re-spam for a while, and thats your chance to drop->jump->f-air. If all else fails, you can jump away from the edge and side b back on, and throw your foot out. Diddy's foot has amazing priority, and will either cancel the gg hammers, or go right through them. (I'm not sure which, whether it cancels or out-prioritizes, there's always the added chance that the hammers aren't out and you hit him normally)