What did the letters GTX represent?

Gas,tires,xtreme
Gas Tires Xtras
Gas Tires and X Wife
Goat Xtreminator
Got To Xcelerate
Grand touring ..... xtreme?
Gran Tour Express
Grand touring experimental.
Grand touring exterminator
Gran Turismo eXecutive
Good Times eXtravaganza
G reat T orque X perience
Great To Excess.

According to Jack Smith, it was a business decision to just take the GTO name since they were selling so well. They couldn't use GTO, so they came up with GTX. He said the X didn't really mean anything, and was just a letter they decided to use.

In Jack's words...

"I was there to become the manager of the mid-sized Plymouth product planning group. When I got that job, one of my first chores was to field something that could compete with that GTO. Thus, the Plymouth GTX was born. Frankly, it was just a philosophical copy of the GTO. We took a premium Belvedere with a premium interior, premium accoutrements and moldings—all that stuff—and added the biggest engine that Chrysler had, the 440 four-barrel. A very strong car! It could easily compete with the GTO.

"And I can tell you how we got GTX, if you want to know. What does that X mean? It's just a letter. We arrived at it in sort of a logical way, but it was a copy of GTO."

or they are all wrong and Ma Mopar actually did print a definition:

https://www.facebook.com/groups/996045710517424/permalink/1275019039286755/