trying to change the head gasket but unable to remove the tube between the intake and exhaust manifolds

If you can apply heat to the fitting I would do that. Maybe use a small butane pencil torch on it for a small concentrated flame. However, only do so if there isn't any danger of fuel leaks or catching something on fire. If so, then use a soldering iron to heat it a little to see if it will free up

Use a screw driver and go around the whole thing trying to pull the tube off the fitting. be careful not to break it. unless it's a metal fitting. then don't worry about breaking it

If it is a screw on fitting put some heat on it and slowly try to turn it. but when you get it turning don't stop because it will cool of and come off even harder usualy