First of all, you should never use gel on wigs. The oils in your hair help the gel take effect and get the look you want, while wigs don't have that oil, as they're fake. Hairspray is much better, and won't ruin your wig.
Take the lock of hair you want to stay flat, and spray it with hairspray. Continue holding it while you hairspray it a good amount.
Next, continue holding the strand of hair, and take a hairdryer to it. WARNING: Be sure before you start that your hairdryer has a cool effect, or that it's on its coolest setting before using it on your wig. If it gets too hot, it'll melt the wig fibers.
After drying it with the hairdryer, you should be able to let it go and it should stay. Be sure to spray the strand of hair at its roots, too.