Propane torch. You will ruin the paint and maybe even the gel-coat in spots if you aren`t REAL careful.
Heat from underside as even as you can an area no more than a square foot or so at a time. When it starts smoking, weight/ hold untill cool enough to touch.
After you get it where you want it then add a bow or brace. I use thin-wall tubing or 5/16th rod.
I`ve straighten some really nasty/cheap glass parts like this. Still looks good 20yrs later