Well, let's do a brief survey of some of the recent commercially available options...
I think this shirt was an M-78 Shop release, and came in three different colors. This is the best of the bunch, a long-gone purple exclusive for Atmos.
Next, a relatively new one from Bandai, with art by Yoshito Sugawara:
I want to love this one; really, I think it would rule the roost...but for the inclusion of not one, not two, but four lines of charmless, titular/copyright-related text right next to Gary's head.
Now, look, I'm just an unfrozen Garamon collector... the mysterious decisions of t-shirt designers frighten and confuse me... but I DO know that this was a mistake... akin to the ruining of a great joke by poor delivery.
""GARAMON" is GARADAMA monster."
Hell, the whole block of text could've been kept, moved to a sleeve or the back, and the end product would've been worlds better.
Here's what I'm sayin':
Anyway... let's look at a similar but superior design. Here's M-78's exclusive (and recently sold out) Pygmon shirt, produced for the 40th Ultra-anniversary in 2006:This is a great shirt. I wish it came in another color besides white (say, pink or tan or a very light green), but that's a minor nitpick. I also wish they'd re-release it, as I slept on it for too long and missed out.
Finally... take a look (though it's a bit hard to see, I know) at Takashi Miike's outfit on the set of Ultraman Max, in this pic taken from August Ragone's ultra-essential Eiji Tsuburaya: Master Of Monsters:Miike in a Garamaniac stylee!
I have actually seen this shirt come up for auction on YJA a couple of times. The design is kinda cool, in a lazy way: basically, it's a shot of a built-up Kaiyodo Garamon kit.
Now, to take it further, into the realm of accessories... you're really too cool for school if you finish things off with this smart pendant:...or this handsome watch: