Quick Question

How do you do a mirror effect in gimp, such as if you wanted to horizontally do one side of a head texture then paste to the other side?

Similar to symmetry art

Select the half of the image and copy it. Then you have to paste it and move it to the other side, giving you the identical half picture next to each other not symmetrical.
Then you need to right mouse button on one of the layers, go to transform and use the 'flip' option.

A quick google search would have helped you too, even faster probably.