scratch that about gates , i had seen it listed (or could swear I did ) , when i was searching through their catalog for greenstripe turbo hoses I downloaded and browsed through their catalogs and could swear in a block listing was the tensioner for the 86-7 3.8 GN and I even searched for price (approx jobber cost 49.00) . it was a great tensioner design but couldnt locate it tonight , just the pulley came up.
GMPARTSDIRECT.com still shows the part