They were bought up by B&M about five year's ago, before that they had been around all my life. I know that they stamp "Flowmaster" on every single muffler they make, so I doubt they didn't have thing's trademarked.
As far as some other, presumably, exhaust company, coming along, and saying "ha! they didn't trademark it, I'm going to use their name", that would never ever stand up in court.
Now, there is a long running suite between Nissan the motor company, and Nissan, some part's stores, where the owner of the part's stores was using the family name, Nissan on his stores before Datsun changed their name in the US. There's another between Budweiser America, and Budwieser Czech, which has been brewing beer since like 1300 or something lol.
But you can't Trademark Squat or grab. You can't just go Trademark the name GTX if it's expired and start building car's, you would owe Fiat Chrysler money, they built that name.