I fished for a while for these and didn't catch a single one, until I moved directly beneath Drachenfall, towards the inside of the cave with the stone monument (North Gustaberg). Almost out of bait, I cast four times and caught four Gavial Fish for my Guild Points. Just a little help if you're having problems. :P
After wasting the best part of an hour trying to fish these up in North Gustaberg (Fishing on bridge @ top of waterfsall. I then realised i was fishing in wrong place. In order to get the location to get these fish you MUST! enter Via Dangruf Wadi. Once inside Dangruf Wadi you need to exit Via J-3 this will bring you out @ bottom of waterfall & just to repeat what Apocolpyse said you need to be directly beneath the waterfall. Hope this helps all you Fisherman out there.
Teleport-altep, grab a choco, and book it over to the oasis of hubol in western altepa. Fishing there with a minnow will catch only catfish and gavial fish. That's the easiest/quickest spot to get to that I've found where gavial fish can be caught.
