I have made jam for a few years with unflavored gelatin. It is great because you can vary the amount of sugar you put in. I still call them "quick jams" because even though you get a good seal on the jar at canning time, I usually make small batches and eat it relatively soon. You don't have to refrigerate all the jars, but don't make too many and eat it within a couple of months. I can't swear to the good quality of jams that have been stored for a long time when using gelatin.

However, if you make the recipe above, you will probably be unhappy with the texture if you like a looser jelly-like jam. My rule is always to use about 1/2 the cups of sugar to fruit (for example 4C fruit = 2 cups sugar) and 1/2 tsp gelatin to 2 cups jam (for 4 cups fruit = two 1/2 tsp gelatin-- or one teaspoon gelatin). I like my jam in yogurt with nuts so I like it to be loose and spreadable. Just a little extra advice.