I just saw the option
--multifile on the manpage:
This modifies certain other commands to accept multiple files for processing on the command line or read from STDIN with each filename on a separate line. This allows for many files to be processed at once. --multi‐
file may currently be used along with --verify, --encrypt, and --decrypt. Note that --multifile --verify may not be used with detached signatures.
What you are specifically looking for is
--encrypt-files and, again the manpage:
Identical to --multifile --encrypt.