The Frog Gun can be bought in the Gem Shop for 5000 gems.
Frog Gun also has 2 Moves in its Moveset, with it also being able to use all 3 types of Haki.
Move 1: Gas arc. Gas arc is the M1 of the Frog Gun, it shoots a green trailed bullet that curves before its landing. The bullet also when it hits a surface or ground will then release a small gas cloud doing flame damage to whatever gets in the field.
Move 2: Gas Fountain. Gas Fountain is the Q move of Frog Gun, this move shoots 7 small gas grenades that affect the area where activated, all landing on the ground and doing great amounts of damage in the 7 increments corresponding to the landing of the grenades.
Note: If possible, Please try and add a video showcase of the frog gun and have it correctly formatted when you can, Thanks.
Frog Gun in the Gem Shop menu