I have a friend who competes in Olifting AND gymnastics. I've been thinking about doing the same thing, but MMA is too appealing. Regardless I use a lot of olympic movements and gymnastic skills in my training as well.
My only warning regarding olifting for you is that your legs are going to get a LOT bigger and that fucks up your leverages for gymnastics.
So you're going to need to get a LOT stronger in your upper body than you normally would as a gymnast
That said gymnast do all sorts of explosive lower body exercises.
Pistols, jumping pistols, weighted jumping pistols, various jumps w/ boxes, jumping rope, pit crawls (no hands), and the simple act of rebounding after a movement (say a roundoff backhandspring) is extremely plyometric by itself.
Those movements alone should allow you to raise your vertical leap.
Keep in mind that you're going to want to not add too much weight to your lower body.
But regardless, olympic lifts are a great idea, bulgarian squats, stepups, weighted jumping squats, weighted jumping lunges, dumbbell swings etc.
Depending how explosive you are already, you made just need to add to your absolute strength. So maximal work with Squat and Good morning variations would skyrocket your vertical leap as well.
It's really hard to be able to help you without you detailing what you do as far as training (and i mean DETAIL-the #'s you put up, and the speed you move at, etc), and seeing you in action performing your skills.
The higherfastersports link is a GREAT place to learn about that stuff... Brad Cardoza has a website (pinnaclefitness), check that out as well. Along with Joe Defranco, he specializes in jumping etc.
Spend a few hours on Google.com too you can usually find some pretty decent info (or at least learn what NOT to do)- any confusion you report back here :-p