yeah, if you're not burning coolant or leaking it, if you're not burning oil or leaking it...
but it's pretty obvious when it happens.. especially if it's all coming out between the block and head.
but here's the situation your looking at to fix it if that's what happened...
(and take into account that you do ALL THE WORK, otherwise, put your car up for sale or part it out and make more money)
well.. nissan engines are overhead cams w/ timing chains.. which means you have to remove, the entire front of the engine.. all the accessories.. alt, water pump, p/s, a/c, then the engine's front covers.
the valve covers and all the air intake and crap around the top of the engine is what you took off first.
then when all the front stuff is off and valve covers are off with all the non essential crap out of the way.. you have to take off the timing chain, idler sprocket, and cams, then you CAREFULLY pull the heads (this involving an indepth unbolting/retorque sequence that has to be followed so you don't warp the heads)
then after an extensive cleaning procedure (giving, you had the engine covered during the long time it's opened) you put a head gasket on it and put it all back together making sure it's timed just as well as the dearlership could do it.
long story short.. it's cheaper to go out, find a replacement engine.. drop that in, and rebuild your original engine while you're hoping the one you dropped in lasts you.
hell, even if you didn't get the replacement engine, go out and spend 400 dollars on shop jacks and engine lift and stand so you can pull the engine and do the headgasket job outside of the car...
it's more precise when torquing the heads.
and i've left a lot out of this... someone could do it easily, but w/o the equipment, it's takes a bit.