I might have it all figured out
...here are the episodes featuring Gobo wearing hats and what they look like:
We Love you, Wembley (has a feather on top; view this at fragglerocker.com)
The Challenge (looks like Mr. Potato Head's Hat)
A Friend in Need (stocking cap)
Manny's Land of Carpets (bulky snow cap with earwarmers; don't ask why...)
Day the Music Died (flannel, taxi cab hat)
A Cave of One's Own (oblong, yellow, and bowl-like)
Invasion of the Toe Ticklers (wears it at the end, Arabian travel hat)
Bells of Fraggle Rock (stocking cap)
Boober and the Glob (meant for Joke Day, a top hat with long purple feather)
Grapes of Generosity (a beanie with a tassel at the end)
Blanket of Snow, Blanket of Woe (stocking cap)
Home is Where the Trash is (a hat like Uncle Matt's)
Wembley and the Mean Genie (a beanie with tassel at the end)
Secret Society of Poohbahs (Poohbah costume, also part of mask, tall & black)
Gobo's School for Explorers (a flat round tan hat)
Scared Silly (like a beanie, but with a front bill)
The Great Radish Caper (upside-down top with a tassel at the end)
Born to Wander (off-white hat, brim all around with beads hanging off it)
A Dark and Stormy Night (Mr. Potato Head-looking hat)
Uncle Matt's Discovery (small Mexican sombrero)
A Tune for Two (beanie with tassel at the end)
Perfect Blue Rollie (beanie with feather at the end)
The Voice Inside (tap-dancing hat)
Trial of Cotterpin Doozer (racoon hat; neat)
How was that? Did I miss anything?