Have 2000 gmc serria no heat when idle give it gas then heat?

First be sure the coolant level is not to low, if the level is okay check engine temp gauge, it should reach normal operating temp (aprox 200 degrees F). If not replace the thermostat. If it does reach normal operating temp feel both heater hoses with your hands. They should both be almost the same temp (hot). If one is hot and the other is warm or cold flush the heater core, it is plugged.

ok well in the summer i blew a heater hose because i put a lift kit in and didn't lenghten the heater house but anyways it broke the plastic piece that rubber hose goes on so i never put the plastic peice back on just by passed it would it be because there is no restricker in there to slow down the coolant would that do it...if not i'lll check the hose's t.m to see what i find...thanks