I’m sure there were similar posts in the past.
If I remember correctly, at least one mentioned that setting password when creating the user was not working (under some circumstances, of course), while updating an existing user changing password was working.
Maybe you could try:
sudo runuser -u _gvm -- gvmd --user=admin --new-password=yournewpassord
And maybe always check:
sudo runuser -u _gvm -- gvmd --get-users
and the gvmd.log