Was Mr Green Jeans gay

The character... No of course not. There are no openly gay characters on Children's TV Shows today so there certainlywouldn't have been any when Captain Kangaroo was on the air.

-If there are gay characters on children's TV today, I feel safe in saying they can be counted on one hand-

If you mean the actor who played Mr. Green Jeans, there is no indication he was gay. If he was that information has been lost to history.