Diesel Bombers - New truck and new to diesels help :C

Hey guys! my second thread on here and its more trouble haha.

So me and my father went splits on this 2005 Silverado 2500HD with the 6.6L Duramax I belive its a LLY, anyways we got a really good price on it cause it needed some work, and one of the things is that the PO said is that it leaked around the water pump a little, but it was okay to drive if you keep checking the coolant and keeping it where it should be.

Though, one thing i've noticed is that the truck will always blow cold air from the heater even if i let it sit outside idling for 20-30 mins and it's still dead cold once i get out to it, but i notice as soon as i start driving it will warm up to the notch between 160 and 210, then if i start opening it up some to say, go up a hill, or pass a car, it will quickly creep up to 210, and then as soon as i let off the throttle it will settle back down to the notch between 160 and 210.... but the truck runs 100% fine, lots of power, starts right up even when its -15c outside ( 5F for you mericans)

Now I'm not a diesel mechanic or am i particularly familiar with diesels but shouldent they stay at a certain tempature? not be all over the place?

Plus i did the head-gasket "test" where you squeeze the hose while the trucks cold and it felt squishy, not hard like they say. And plus i noticed even when the trucks cold, if i go to open the coolant bottle it will bubble and hiss as if the system still has pressure? I've read that it could be the thermostats, or a number of things..

But we did buy a new water pump and we will be having it installed next week, any help on the "all over the place" temature gauge would be great.

Also, heres a picture of said truck which i think we stole for $3,000
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