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.
Looked up 'gelatin strength' in Google and found this link. Very interesting.
There are two types of gelatin: A and B (I have no idea what the 'isoionic point' is).
Anyway, in the shop, I guess you'll find approximately the same 'strength' gelatin, independent of the format.
Normally, the gelatin sheets are 2g and the granular gelatin is packed in 6gr envelopes. 3*2=6.
6g of gelatin (1 envelope) will bind 500ml water for jelly (as per the instructions on the envelope). Substitute water for other liquid and you are set to go.
Edit: Personally, I don't think the precision is too important. This weekend I added too much water to a recipe (500ml) and the result was nearly perfect. That is, it didn't collapse but was hard to ration.
Also - Changed gr to g.
Best Answer
Not guaranteed:
If you don't mind just wasting the Jello, or winding up with Jello as a snack*, you can try this to kill some time:
Try to dissolve the whole thing in very cold water. Maybe even ice water. The water must stay cold for this to have a chance.
The sugar should dissolve at least some, and with stirring a little more. The sugar in the packet is superfine compared to table sugar if my memory is correct.
The gelatin will absorb water but not dissolve. so it should precipitate to the bottom.
Decant the liquid off. take care to keep everything cold.
Then maybe try it all again to further purify.
This should reduce the percentage of sugar in whatever you have.
The flavoring is probably citric acid and esters, so they are probably very water soluble.
The problems:
You will not know precisely what percentage of whatever is left is gelatin vs. other stuff.
You will not know how much "dry" gelatin is left as it will be hydrated now. So for example, will not know how much of your gelatin = 1 tsp of dry gelatin.
The chemistry Taking advantage of relative solubility of the components in cold water. This site says gelatin will absorb but not dissolve in water under 40*C (about 104*F).
*If it fails, just heat the whole thing up in a sauce pan until all dissolved (do not boil), stir well, and then pour into bowls and chill.