In Zul'Gurub, south of Arlokk by the riverbank, is an abandoned camp with a bunch of overturned jars and chairs. One of these contains Nat Pagle's Measuring Tape. Take this tape and return it to Nat Pagle in Dustwallow Marsh. After returning the tape you will be able to purchase a Mudskunk Lure
Enter Zul'Gurub with a Mudskunk Lure and look along the waters for a "Muddy Churning Water". Fish these pools with at least 350 fishing skill, and catch 5 Zulian Mudskunks.
Head just east of Thekal to Nat Pagle's fishing point by the water. Use the lure at this location and Gahz'ranka will be summoned. Note that lure is loaded and cast at the same time, which summons Gahz'rank. You can only summon Gahz'ranka once, so if you fail to kill him you do not get another attempt
The fight is pritty eazy, hes basicly a bigger version of the hydra in ZF, give tank some argo time and then DPS him down, expect periodic knockbacks and kicks that toss you up fairly high.