Medix uses your GMC number because it is a unique identifier and should be known to members. Self-chosen passwords help prevent unauthorised access because your GMC number can be found on the Internet.
If you want to save the effort of typing your GMC number and password each time you login to Medix, just tick the "Remember" box at login. This means that anyone with physical access to your personal computer (PC) can login to Medix, but it is difficult to see problems arising from this.
You can do the same when editing your registration details for Medix. This could create problems if you do not have physical security for your PC. If pysical insecurity is a possibility, save only your GMC, not your password (leave it blank when you tick the "Remember" box). This will prevent unauthorised changing of your details.
Medix questionnaires normally offer themselves with your GMC filled in. You must type in your password, however, to prevent unauthorised response.