Yes, Cloud Storage Gateway is compatible with Portable Operating System Interface (POSIX) semantics.
A compatibility test by using the pjdfstest suite shows that Cloud Storage Gateway (CSG) provides up to 96.98% compatibility with POSIX semantics.
Note
Main incompatibility aspects are:
Object name length (The object name must be 1 to 1,023 bytes in length).
No support for hard links.