... Actually, I connect it to my iPad Air's headphone jack and the GTX works great. I was hoping to do the same with the laptop but it's headphone jack performs unacceptably. That's why I was hoping to use one of its USB ports instead.
Has the MBP's headphone jack degraded over time, or was it always flaky?
How often is the MBP moved? How many insert/remove cycles does the MBP's jack endure over the course of a week? Would the USB port need to endure a similar number of cycles per week as the headphone jack does?
I'm asking because 1/8" jacks simply aren't designed for long life, at least not the compact ones in typical computers. You may be able to find an independent technician who can replace the jack, or maybe not.
Sadly, USB A receptacles (the female ports on the computer) are also not designed for long life (defined as insertion cycles):
https://en.wikipedia.org/wiki/USB#Durability
... The newer Micro-USB receptacles are designed for a minimum rated lifetime of 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. Features intended to accomplish [this] include, a locking device was added and the leaf-spring was moved from the jack to the plug, so that the most-stressed part is on the cable side of the connection.
So if the MBP gets moved around a lot, and its USB port endures lots of insertion cycles, it will eventually fail, too. It's likely to become intermittent before it completely fails, but an intermittent USB port is as useless as one that's completely failed, because no USB devices will be able to retain a data connection.