Gator gar are actually apex predators and also good eating, once you get through the scales. Large fish take most of a century to reach that size, and so they are very vulnerable to over harvest.
Bowfising has a very high mortality rate. There is no catch, photo, release option. Those people overconsuming the resource and wasting it will exhaust the population of mature gator gar and put a huge dent in their ability to reproduce. Gars only tend to reproduce when there are flood conditions or a long stretch of flooded shoreline brush and vegetation. They may go several years without reproducing in a time of drought.
Imagine if a fair number of hunters started ahooting trophy bucks, taking a picture, then dumping the deer in a ditch. For fishermen, these bow hunters are basically doing that when they harvest many large gator gar, then don't even harvest them (a legal requirement, even for taking non-game fish or even live bait in Texas). Wasting the resource is unlawful.