How to make gill fishing?

[LineageRazor]

I've found that Sharks seem to be pretty reliable for profit. Alchemists use them to make Shark Oil, and a lot of important crafting recipes use Shark Oil. Hammerheads are pretty easy to fish up with Shrimp Feeders, and I've caught a ton of Silvers trying to complete my fishing log (I'll catch you one day, Titanic Sawfish!!!).

[Shirai]

(I'll catch you one day, Titanic Sawfish!!!).

Grrrrrrrrrrrrrrrrrrrrrr. This!

And yes, sharks are a good source of income.
Other then that you can go for Fullmoon Sardines, Red Choral and Black and Whitelip Oysters at the Sapsa Spawning grounds.
They sell for less than sharks, but between them you will have a higher catchrate and greater variety of items, and you can get the bait (Northen krill) not too far from that fishing spot either.
Just be sure not to Mooch the sardines for Nautilus', those are worthless.