In case someone else wants to try this setup, it is a bit tricky because s3fs expects to use AWS, and the Minio server that the s3-gateway is based on does not work like that. On MacOS I got it working with this commandline:
s3fs /mountpoint -o bucket=test1 -o profile=tardigrade -o url=http://localhost:7777 -o use_path_request_style -o sigv2 -o auto_xattr
The profile= is a regular awscli profile (for the username/password) the auto_xattr is required to allow Finder on MacOS to show the contents.