Is it safe to use a counter as IV for AES256-GCM What if I will use 96-bit integer counter starting from 0 as IV for AES256-GCM cipher. Is it safe? Is there any other ways to ensure uniqueness of IV for GCM cipher?
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.