It makes no difference whether you use sheet or powdered. Just make sure to use the right amount. It should be given on the package, like "this sachet is enough for 500 ml liquid". You can try to use less than indicated for a softer texture, but if you don't use enough, it will stay liquid, so you may need to experiment until you get it right.
You can't bake gelatin. It is a protein and denaturates at 50°C. The proper way is to bake your pie or fruit cake as always, and after it is ready, pour the gelatin containing liquid over the fruits and wait for it to congeal. (I am assuming here that you just want a gelatin glaze over a complete pie. If you want a complete gelatinized filling, everything below applies. Bake a shell, put the fruit in it, then fill it with the liquid).
The liquid itself can be pure water, or water syrup, or, if you used canned fruit, you can catch the liquid from the can and strain it. Then use it, pure or diluted with water. Sugar of food coloring don't interfere with gelatin, use them as you see fit (I think it's popular to color the gelatin on strawberry cakes a light red).
The mixing is very similar to panna cotta. First, bloom your gelatin by putting it into a cup with some water (1-2 tbsp) and leaving it for awhile. It should soak up the water, use it while it still looks moist, before the granules stick to each other. It usually takes around 5 minutes. Meanwhile, warm your liquid on a very low setting. It is best to use a thermometer. Dump the gelatine into the liquid while still on the heat. Stir until dissolved completely, never let it get too warm. When dissolved, remove from the heat and pour immediately.
There aren't many things which interfere with gelatin, but some fruits (or the enzymes in them) do. These tend to be tropical fruits with at least moderate acidity, like papaya, mango, kiwi and pineapple. If you pour gelatine on them when they are raw, it will have trouble setting. Cooking them should help, but I don't know how long or what temperature. If they are canned, it should be OK. Bananas are not a problem, even when raw.
And because you said you want to make it "professional": If you are talking about a thin glaze, gelatin is not your only choice. You can use starch as a thickener, many professional bakeries do so. It eliminates the possibility of getting the right texture of the glaze (which will be rubbery with too much gelatine and half liquid if you don't use enough), and maybe you'll feel more comfortable with it.
Best Answer
First of all, adding more gelatin is not something I would try. If the problem were too little gelatin, you would have a result that is uniformly too-soft or even liquid, not a difference between top and bottom. Also, adding more "just to be sure" is a bad idea in general, since extra gelatin makes for a rather unpleasant rubbery texture and also reduces aroma.
You don't give much information to go on, but I will assume that you meant
to be literally your full process. If that's so, then you didn't properly use the gelatin, and it is normal that you are seeing weird results.
The proper way to use gelatin is to bloom it.
This is the basic process for gelatin. It has a few variations (e.g. when you are making a whipped-cream mousse), but for your case, this is what will give you a proper juice-based dessert.