I lived semi near to the colorado and llano river, i heard theres gator gar in there but how do i catch them. What size reel would be need for them? i dont think my ultralight reel would work

If you are looking to catch 4' and bigger gator gar you need some much bigger gear.
100# > line, 300yrds minimum, a sturdy rod, and a lasso.

Here's a video that tells you everything you need to know.
It's broken down in chapters so you can skip around.
https://youtu.be/Xw3Gadue79g