Glue gun glue is much too stiff for this. How are you sewing if a sewn patch is coming off? Here's how I do utility mending: http://www.picturetrail.com/sfx/album/view/24395076 For an actual hole, I'd do much the same thing, starting with a scrap of knit interfacing to "bridge" the area under the real patch.
No the glue gun is not usually used for this type of repair. There are fabric glues that will work better, or you can iron some iron-on interfacing to it with a new piece of fabric in front and behind the hole, or purchase an iron-on fabric repair kit. All of these items may be found in a fabric store or possibly in stores that carry sewing notions. This video may also help: