Also, how do I put gel on my hair after flat ironing it?

When you straighten your hair with a flat iron it usually holds its straightness for at least a few days, about three whilst washing your hair every day.
Adding water will probably make it go frizzy though.

You can get waves if you use gel and scrunch your hair.
You have to take sections of hair in a hand covered with gel, grabbing them from the roots, and squeeze outwards.
The smaller the section the tighter the curls.
I know for a fact that this works if you have short hair and want beach waves and supposedly it works to give waves to longer hair too, but I can't say for sure.

The 'flat' effect you get when using straighteners or blow drying is caused by manipulating the hairs hydrogen bonds which can easily be reformed by heat or water.
If you add product or water after applying heat you are just reforming any bonds you have changed with the heat source.

If you wish to achieve a straight look with extra hold you are best to apply product to damp hair before drying with a hair dryer, when dry use the straighteners on your hair in small sections. There are also some good humdity sprays on the market that you can spritz as a finish to tame any lasting fly-away ends.

Not necessarily itll just turn to like a stiff wave thing at most. If you want to make it curly again try washing it.
I've heard of straightening gel but never tryed it...