Not sure GAM does outside endorsements, when I wanted to do a piston commander endorsement I had to go to Hobart.
Problem is do you have access to an aircraft? If not I suspect that will be the stumbling block. I do know there is a chap in Essendon not attached to GAM who can do it but aircraft becomes the issue.
Go to the twincommandergroup.com forum and ask the question there. The people on that forum will give the right answer.