I'm sure that a LOT of it is marketing, but, if you go in and read the descriptions, there do appear to be some pretty significant differences.
The GTX has a LOT of bells and whistles stuck into a generically described cabinet using a generic speaker. Reading through the ad copy, it looks to me like an amp that they packed a bunch of stuff in, some things I would never use (yes, I'm basing this on what I would or wouldn't do with it). It's an amp that's trying to cover a lot of bases so that you could find an acceptable sound no matter what you needed.
Looking at the Tone Master amps (specifically the Twin), it appears that they're trying to make a fairly faithful non-tubed version of the original amp. It has vibrato and reverb. It has the the Bright switches. All of that is on the face, not something you have to go into a menu to change. They actually specify that the cabinet is made out of pine. They use a specific Jensen speaker and not one that's just labeled as "specially designed." Looking at the back, it also has an attenuator built in (that has setting between 1w and 85w). This one isn't trying to be a jack of all trades, it's trying to do one thing, and it appears to be trying to do that one thing well.
Personally, if I was looking for a Fender amp to gig with, I'd skip the GTX line and look at these. I like the simplicity of the control face. I like the fact that there are no menus. I like the fact that it's the same thing all the time. That said, if I was out to buy a Fender amp to carry out, since they're only $200 more, I'd skip the Tone Master and look at the real thing. For the price difference, the real tube amp would probably be the better deal, but that built in attenuator could sway me towards the cheaper one.
And, to be honest, since I'm not looking for an amp to gig with, if I was out to buy another Fender, I wouldn't be looking at any of these. I'd be looking for an old Vibro Champ.
YMMV