doing your valve cover gaskets shouldn't have anything to do with your heat.
first thing to check is your coolant level.
start your engine and watch your temp gauge. does the temperature go up to normal? it may take a while but you should be able to get up to around 200 degrees. once the engine temp is up, what is the vent temp? if you don't get heat then, check the two coolant hoses that go to the heater core in the engine compartment. Are they both hot or is one hot and the other one cool? If they both aren't hot then the hot coolant is not going through the heater core. Back flushing the heater core may make it so you have some more time before you have to replace the heater core.
If both hoses are hot, you should see if your blend door is working. If a scanner is available, scan your hvac and see if you have codes for the blend door. the blend door motor may be defective.
On the 03 buick, I have to move what I believe is the heater core to replace the rear valve cover gasket, there are two l shaped plastic pipes connecting into the motor, when I removed it, collateral leaked out, and I refilled it. It is only after replacing the gaskets, did I not have heat, that is the only thing I can think of.
you could have a big air pocket in the heater core if you took the heater hoses off. It could eventually bleed the air out over time or you can have a shop drain the coolant and then evacuate the air and let the vacuum pull the coolant back in. If that is what the problem is now.