The user should not be gendered, the error message :

User cannot add himself to a disabled group or a group with disabled GUI access.

should be changed to something like

A user cannot add themself to a disabled group or a group with disabled GUI access.

OR

You can't add yourself to a disabled group or a group with a disabled GUI access.

Steps to get this message (on master):

1. Go to Users->User groups;
2. Press Create user group;
3. Fill form:
   1. Group name = "test";
   2. Users = select your current user;
   3. Frontend access = "Disabled";
4. Press "Add" button.