Is it safe to use a counter as IV for AES256-GCM
See page 20 of the NIST recommendations (28 in the pdf, 20 on the page) - basically this is the deterministic construction (assuming you're doing less than $2^{64}$ messages), so yes this should be fine - assuming (!) that you don't reuse IV (even across restarts / crashes).

Alternatively, you can use the RBG construction, and use a secure random source to generate the IV.