St. George will be hot, very hot. (You could get lucky with a cool day though.) However, Gooseberry is much higher than St. George (~5200 ft. vs 2700 ft), so it'll be a bit cooler than St. George. You should be aware that it'll take 45' to an hour to get there from St. George, so take that into account.
That said, I've ridden it in the summer, and I'd do it again. If St. George is in the 100's, Gooseberry will probably be in the 90's. One difference from Florida, there's not much between you and the sun - you really feel it when you're not in shade.
To navigate, check out the info here:
http://www.utahmountainbiking.com/trails/
It's very easy to navigate the South Rim. I'd recommend doing that first, and if you're overheated when you get to the point you can come back to the trailhead on the road that goes down the center of the mesa. This also gives you the option of doing Hidden Canyon (recommended - do it as an out and back from the South Rim or can also do it from the road). On the return, the short yellow dot sandpit bypass trail is very good.
The north rim gives more opportunity to get lost. A GPS is helpful but by no means necessary. As long as its not getting dark and you're not overheated you can pretty easily find your way back to the road that runs down the center of the mesa. Also, if you end up at the windmill, you can just take the roads back to the trailhead.
