All just my opinion.
Grognard wikipedia entry
For Traveller anyone who started with CT can be considered a possible
Grognard in the gaming usage of the term. The extreme end is an insistence on CT as the One True Holy Way to Play Traveller to the damning of all other paths to Traveller. This may or may not exclude houseruling. There are degrees and variants on this, such as accepting some parts of MT and DGP’s works and TNE’s FF&S but still bolted onto what is essentially a houseruled CT. There are also MT grognards and with 1248 in the works TNE grognards might be possible. For the most part though anything CT is seen as the only way to do things and is the only thing deemed Traveller.
Gearhead wikipedia entry
Gearhead is the car term taken to Traveller vehicle, gadget, weapon, and ship design. Traveller gearheads focus on the design minigame to at least a significant if not exclusive degree. Variants include rockhead for planet designers and there’s likely somebody who cranks out species. While a curio and a byproduct of Traveller rules any religious wars are usually within the ranks.
Canonista referrers to taking only those parts of works as officially approved by the High Holy MWM as viable in any discussion or playing of Traveller. Anything else is false teaching and Heresy. Again there are variants and degrees but it is more of a charged term than Grognard. I don’t use the term Canonista lightly as I see it as a negative only.
Someone can have elements of all three and usually at least two are present especially if a Grognard uses only the OTU. This doesn’t have to be the case though since the OTU developed by pieces and wasn’t present in the initial books.
Technically I suppose I'm eligible for the title grognard, having played Starter/TTB back when it was in print (I remember the ship illos, misjumps, chargen, and that's about it). However I feel there is no one true way to play Traveller and don't really care what one uses to play it or what setting is used, especially if it gets in the way of playing an enjoyable game. I’ll use whatever design sequence or fudge to generate a design or just use someone else’s.
I like to keep religious devotion to matters of religion. For me that doesn’t include any game. Not even my beloved Call of Cthulhu. The Mythos shouldn’t be set in stone and memorized, nor did Lovecraft set his stories in the 1840s, and the new World of Darkness core book or Unknown Armies should work just as fine as BRP.
As always YMMV.
Casey takes the bicorn off