We don't have/use sure gel is Australia. When I make jams I add lemon juice and cook it until what ever fruit I am using softens then i will bring it to 220F which produces a nice rolling boil which can't be stirred down. Cook it at this temp for 5 minutes then turn off wait a few minutes (up to 5 mins) then jar as usual.
I should say I usually do a sheet test before I jar it just in case it needs a little more time although usually this is enough.
Maybe1day