I agree with Jan. There have always been people traveling solo on any of the tours I have taken. And I traveled solo to Russia last August. I am traveling solo again in June, to Ireland this time. I was not able to get the first date I asked for because they didn't have any more slots for solo travelers. It's understandable that they, as all tour companies do, limit the number of solo travelers.
Traveling solo with GCT is much better than with any other companies I have investigated. They don't charge the single supplement on most of the land tours and a much reduced supplement on the river trips. They still have to provide the buses, program directors, local tours, accomodations, etc.
I find it hard to believe that when they couldn't give you that reservation they didn't offer you another date.