Yes you need canti levers (BMX levers are fine too but maybe not that great for the front canti).
BMX U-brakes will work but bear in mind there are some (such as the AD990/996/Hombre etc, maybe more) that were designed for front mounting on a BMX so might have some odd looking (to those who haven't seen them) cable mount points. They can be used still but you might need a bolt on cable barrel for one side of the straddle.
Also make sure the angle of the arms fits your U-brake cable mounting direction - on a GT I think you are always better with the crossover cable as it increases the angle into the brake.
Here is the U-brake on my Tequesta:
That is a Dia-Compe Hombre, a later version of the classic AD990 (which had stud type brake pads). The improvement on the brake in the packet included green tension adjusters (Which I stripped to silver) which were CNC'ed for a lower profile for chainstay brakes and road style pads which were easier to adjust.
The straddle cable is wrapped round the seat tube - you can see the LHS has the normal straddle "pull tab" mounted in it, but the RHS has a bolt on cable barrel. If you imagine on a BMX fork the cable would be routed down the steerer tube so you can spin the bars so the cable would come up from the bottom - see this pic of my WTP Pony (why o why did I sell this!!!!!!).
So thats why you need the clamp on thingy. I may have one you can have for free if you need one.
Some of the brakes come with a regular cable clamp on one side instead.
You can pick up Dia-Compe Hombres really cheap on ebay sometimes, but try Alans BMX or somewhere like that for one instead. They will be better than a Shimano MTB U-brake any day of the week.
The new type of U-brakes probably won't work any better on an MTB than a Ad990/Hombre type but do come in fancier colours. I wouldn't bother with the £40 CNC ones personally. Exposure do some nice cheap coloured ones.
The one on my Tequesta has got "Dura Ace" style road sticky pads on - man alive that brake worked well!!!
I can get some links to some I would recommend (brakes) when I get home.