Assuming the pants are HQ...

STR is already capped
VIT is already capped
Crit is already capped
Determination, it can hold 5 more
Skill speed looks like it caps at +19
Accuracy looks like it caps at +27

I wanted to know if it's possible to get gryphon skin equipment that's HQ ilevel70 and then stack my primary stat of STR on it
No, the primary stat(s) on HQ items are always capped.