I have this as a research item; I've heard Gary, Ernie and Brian from different people.
My opinion:
The encounters, especially in the pre-1980 versions, sound a lot like Gary.
I think Ernie actually worked on the geomorphs and not on the M&TAs.
The 1-100 texts of monsters and treasure strike me as something Brian Blume would do. Compare to the readouts in the Rogues Gallery, and the nature of the Boot Hill rules. The guy had an Arnesonian fetish for percentiles.
I'll post an update if I find something definitive.