To actually catch a Gravid Bowfin in Monster Hunter Wilds, you'll need to use your Fishing Rod. Do not use the Capture Net, as it won't count for the mission. By now, you'll know how to use the rod, but with the Gravid Bowfin, you need to equip the Emerald Jitterbait for the best chance of catching it.

Here's a list of Monster Hunter Wilds fish and where to find them, including the Goldenfish, Gravid Bowfin, Whopper, and Grand Escunite.

I found the cave system in Areas 14 and 15 in the Windward Plains to be the best location for catching Goldenfish in Monster Hunter Wilds, but you must catch one with the fishing rod equipped with the Golden Bughead lure to complete this side quest.