There are a number of predatory fish that you could keep successfully in a 30 gallon tank. A lot of it depends on just how much effort you want to put in to feeding such a fish.
A pair of Leaf Fish (Monocirrhus polyacanthus) would do wonderfully in a tank of that size and they are very fascinating predators. HOWEVER....these fish generally won't eat anything EXCEPT smaller live fish so you would have to be willing to make a committment to feeding them up to 1000 small feeder fish per year!
If you can find them, the smaller members of the genus Ctenopoma, like C. ansorgii or C. acutirostre are quite voracious predators on smaller fish but at the same time can often be coaxed into eating frozen foods as well. They are also fascinating fish that will fit into a 30 gallon tank.
I must have missed the part about a 'gobie face' but all of the species of Polypterus will eventually get too big for your tank and they're not very spectacular predators anyway...being as they do most of their hunting at night. Don't get me wrong....Polypterus are my own personal favorite fish but they're probably not what you're looking for.
Exodon paradoxus, a member of the tetra group, is a nasty, greedy scale eater that will sometimes take entire smaller fish. They are beautiful fish and are often kept by people who like piranhas (though usually not in the same tank).
Depending on their legality where you live, various dwarf species of snakehead, like Channa gaucha can be kept successfully in a 30 gallon tank. These are VERY predatory fish and are now illegal to ship across state lines (even the eggs are illegal to ship). Check with your local fish and game department before considering these fish.
There are a number of other possibilities....a little more information about precisely what you are looking for in your fish would be helpful.