The Turnip itself won't knock the grenade away - most likely, it will be Peach the grenade hits that causes it to bounce off Peach and thus causing it to stop
Unfortunetly, by Glide Tossing towards him, you put yourself in the dilemma of the grenade being in the air but near to the ground. You're close enough to try and hit Snake but the grenade can go off and knock you out of say, your Float
I obliterate Snake's who constantly use Grenades ---> Roll drop out the way with Sonic but with Peach, it's much harder
An important thing to remember is that when Snake has pulled out the grenade, he is very limited to what he can do. He's either going to have to throw it which is fairly laggy or he's going to have to put up his sheild/spotdodge/roll, all of which are fairly punishable. Or he might decide to cook the grenade and throw it but if you're close by, throwing it won't be the best option as he might get caught in the blast of the grenade as well
Watch for how long the grenade has been out for. When Snake pulls out the pin, they will go off in about 3 seconds. If you're going to Z catch it, your best bet is to only catch the ones the have ALREADY hit you so it doesn't mess up your chances of the grenade bouncing away from you (and ultimately exploding in your face). Catching with aerials is risky as you can set the grenade off early if you hit it. Toading grenades is very very unreliable as the counter can be set off when the thrown grenade hits you and timing Toad with the explosion is very dodgy, especially since most Snake's will be well clear of getting hit by any spores. If the grenade has been out for quite a while, don't attempt to pick it up and throw it because by the time you do, it will already have exploded in your hand. If you do catch one, use it to your advantage! Throw it at Snake so that it will explode just as it reaches him
...I've said to much >< Carry on