Well, you have a few options. Ill list what I use (keep in mind I have at my disposal 4 GoPros (Hero 3+ silver, 2x Hero 3+ Blacks, and a Hero 4 Silver), as well as a Mobius action cam, and a 3DR SOLO UAV for use with a GoPro).
1: Headband. GoPro makes one that is decent, however it tends to bounce a little. I use this when sniping, as I wear a hat when sniping.
2: Helmet mount. I just put one of the adhesive GoPro mounts on the NVG mount - its worked fine so far. Companies also make proper GoPro -> NVG mounts, but I use the basic setup. (I know you said you dont use a helmet)
3: Chestie mount. Another GoPro mount - one of my least favorites as it can be blocked by your gun, and you have to be facing a target to see anything worthwhile.
4: Buffer tube mount. I use a GoPro handlebar mount on the base of my buffer tube on my ARs. This gives the view a very COD-like view, and is one of my favorites.
5: Barrel mount. Again, using the handlebar mount, you can either put it at the tip for a view of yourself, or towards the base of the barrel for a forward view.
6: Shoulder mount. I dont use this one, but watch Novritch to get an idea of this one.