I like to add hydrolyzed proteins to my FSG. Here are few good websites that sell them.
These websites also sell preservatives that can be added to FSG. If you choose to use them, please read up on how to use them safely. Very small amounts are used! I use Liquid Germall Plus in mine, and add about 1/4 tsp to about 16oz FSG.
I've never used grapefruit seed extract, so I can't offer advise on that. I do use citric acid in mine, but just a pinch to adjust the pH. Citric acid is not enough of a preservative to keep nasty stuff from growing in your gel on its own. Keep you FSG in the fridge until you decide if you want to try a stronger preservative.