The gnome-keyring flag toggles libsecret support and pulls in app-crypt/libsecret which pulls in virtual/secret-service which depends on gnome-base/gnome-keyring or app-admin/keepassxc. It will probably support kwallet in the future too. The flag should be renamed IMO. mate-extra/mate-power-manager uses "libsecret" and x11-plugins/wmudmount uses "secret" for this purpose.
At the time of adding it, that was the standard. Imo this is a call to find a proper global IUSE. See also the fact that wmudmount and mate-power-manager couldn't even settle for a common flag around libsecret. There is also app-containers/docker-credential-helpers with gnome-keyring and the following description: "Build secretservice helper which works with gnome-keyring or kwallet using D-Bus." Library names as IUSE should be avoided so a name describing the function should be found.
+1 on a proper global name, I think at the time I renamed USE flag for wmudmount when upstream switched from gnome-keyring to gcr+libsecret (for same functionality) so I just picked "secret". But this can be renamed to a proper one no problem
