Plums contain enough pectin to turn into jam with only sugar and maybe some lemon juice (for acidity) added, though if you're doing this the first time you probably want to add extra pectin. Over here that's generally done by buying sugar that's laden with pectin, in other places you buy the stuff without included sugar, I recommend to search for recipes on your local internet. If you have a plum tree in front of your window someone in your country will know how to turn supermarket ingredients into jam.
Gelatine in jam is, IMNSHO, a travesty. You're not making jello.