when should the intake gasket be replaced on a GM motor?

when it goes bad. is the obvious answer. like the guy already said, if water is leaking out, or anything else.

i just smell antifreeze evey now and than ...nothing else runs well...

then there is your answer, it's hard to find the leak because it usually wont leak unless the engine heats up, but check to see if that is where it's coming from. I find it strange that that Is the first part you would go to, to be bad. There could be much easier and cheaper solutions than going for that gasket. make sure your water pump isn't leaking out of it's weep hole. on larger American engines, the water pump has a tiny little hole drilled in it and then it is sealed, once it rises to a high enough temp or bearings start to go bad, coolant will very gradually leak out as a early warning sign that the water pump is going out. check heater hoses, upper hose, lower hose, especially where the fittings meet. Also try and see as much as you can around the radiator. see if it's leaking. start the truck with the radiator cap off, run the heater so the thermostat will stay open to circulate the entire system, and keep room temp water nearby in case a little leak turns into a big leak. just be patient and take your time. don't get yourself sucked into the fan and don't burn yourself. don't try and put the radiator cap back on till the system is completely cooled. that' all I can think of man

I was told after 100,000 miles it should be changed,.crazy..lol