2006 Sierra very little heat

[Sierra heat]

Hey guys, I recently bought a 06 Sierra and I am having a cooling/coolant problem. I started noticing that I wasn't getting much heat and I checked the overflow and it was pretty much dry so I added more and it seemed fine at first then the same. My temp gauge is only at 1/4 (between 75 and 100), is that normal?, it has a 4.3. Any info would be greatly appreciated

 

[03DumpRunner]

I had that problem in my van and it turned out to be an air lock in the coolant system