No heat on my 1996 Yukon

What is your question?

I am having an issue with getting no heat. I have noticed over this winter that it takes for ever for my Yukon to defrost, even when I finally get it to defrost a little bit, the air coming out is not warm. I have checked the fuses and found no issues there. I have noticed that the temperature gauge is barely over 100-120. Is this a thermostat issue? Blower issue or do I have bigger issues like my head gasket? Any suggestions or ideas of troubleshooting would be gratefully helpful. Thanks

What makes this problem better or worse?

Can’t figure anything that makes it better or worse except if it’s a nice day and don’t need heat

How long has your 1996 GMC Yukon had this problem?

Last two winters.