I use gimp and I'm new to it. If u i make the size of the picture 512 by 512 pixels does that mean it's a 512 head texture or is it only 512 on gimp? It might be a stupid question but I want to confirm before I sell things.

If it's 512x512 and you upload the image on "512 head texture" item then it will work exactly the way it looks and yes it will be 512 quality texture.
Tho if you have 512 image and upload it on "Head texture" (so 128 tex), it will resize and scale itself down, resulting in noticeable quality loss (it becomes blurry). If you have 128 image and upload it on 512 it will look just like 128.

Tl;dr if you make 512x512px image in gimp, it will work as 512 texture.