I'll try my best to explain a few exercises in English.
1) You could work with rubber bands, or weights doing extension's, flexion's, ulnar and redial deviation. The same exersises can be done with help of a coach applying 'pressure' in the opposite direction of the movement done by the gymnast.
2) Without weights or bands: From a hand and knee position, hands flat with fingers to the front, lift your palms off the floor (fingers remain on the floor). Add as much pressure by leaning more on the hands (and less on the knees). After some exersise you would be able to do this from a push up position.
3) Same position as in 2 but on your fists with palms opposing. Open your fists so the back of your hand touches the ground, than close them. Again, apply as much pressure as you can to finish your set (12x for example)
4) Push-ups on the fists, with elbows far to the side, rolling from the top of your fists (in support) to the side of your fists (when you bend).
5) If you're a bit handy you could make next 'device'. Take a small bar and slide a tube over it. Attach two cables (small ropes) on the side of the inner bar so you can hang it somewhere (ceiling, high bar?). Then attach (tape) a rope with a weight on the end in the center of the tube. Now you can turn the outer tube backward or forward so the weight gets up to the bar. Great way to train the wrists...
Hope this makes sense?
These are just a few ways to train wrist strength.
Make sure you build it up, because when you start some of these exersises can be hard.
Good luck!
