Gamma is an overhaul mod for Anomaly, which is a standalone modification of the original CoP running on its own engine. You aren't actually installing it over CoP so normally you'd be fine, but for whatever reason the GoG necessitates GoG ownership of the games despite Anomaly itself not having any such restriction. Get the launcher agnostic version from their discord server instead.

Addendum: They have recently, as of this month, put up an independent github page with an installation guide and the downloadable files as well.