How to encrypt files using AES256-GCM cipher under Linux?
LibreSSL supports AEAD ciphers, including aes-256-gcm:



$ openssl enc -aes-256-gcm -nosalt -p -in file.in -out file.out
enter aes-256-gcm encryption password:
Verifying - enter aes-256-gcm encryption password:
key=A744E1091C25BABD36B50E40FB8D311A672722729CEA6E217AD9FA8AF23CAF57
iv =BDEEA37B93BB989C6C40665B


If you don't mind writing your own software, there are plenty of crypto libraries supporting AES-GCM, such as OpenSSL itself (even if not available from the command line tool). Libsodium also provides AES256-GCM and may be easier to use.

That being said, using a common and trusted tool such as GPG would be way safer than implementing your own software, even if the resulting files are not encrypted using AES-GCM.