I assumed that since the in car heater was working during my return home, this meant there is still sufficient coolant in the system. As far as I'm aware if coolant becomes low the in car heater would have given cold air.
Depends on the exact car but generally yes this is correct - at the very least it indicates that there is enough coolant in the system to have some passing through the heater matrix. It doesn't always mean there is sufficient to provide adequate cooling for the engine though.
I also drove slowly(under 3000 rpm). I assumed even if coolant is not there, so long as you drive 2000-3000 the engine won't overheat, its only if you rev higher.
Not quite - what generates heat in the engine is when it is under load (which is not always the same thing as high revs), and driving slowly can actually be counter productive as this will significantly reduce airflow to the radiator and engine bay which is badly needed to cool the engine when you are marginal on cooling. This can be seen when cars that have cooling system issues overheat in slow traffic or stationary. Ideally you want to travel as fast as you can with as little application of throttle as possible - lift and coast as much as you can!
I kept my eye on the temperature gauge and assumed that so long as it doesn't show overheating into the red area, which it didnt, i should be safe.
Not always - the "temperature gauge" in most cars is actually a measureing the temperature of the coolant at a certain point in the cooling system. If there's no coolant, or if there is an airlock you could see an artifically low reading that doesn't reflect the reality of what the engine is experiencing. If there's still coolant in the system and you're seeing a "normal" reading that's encouraging but not cast iron.
Were my assumptions correct, if not please explain why. Also could head gasket now be gone or would that only happen if your temperature gauge goes to the red region?
It's possible that the head gasket could have gone but in my experience this would be unlikely from one short period of low coolant - although it very much depends on the engine in question as some are less robust than others. My old car (S6 with a V8 engine) had a thermostat failure and I limped some 20+ miles (with multiple stops to cool down and top the boiled off coolant) and the headgasket was absolutely fine - try that in something with a K-series engine and the results would likely have been a headgasket failure.
Besdt course of action now is to avoid driving the car (if possible) and get the leak tracked down and fixed. Once that's done get the system emptied, filled with fresh coolant, bled and watch it like a hawk for a week or two. Keep an eye out for any of the classic signs of headgasket failure:
- erratic temperature readings
- steam from the exhaust
- oil in the coolant (looks like a brownish floating scum in the coolant expansion tank)
- coolant in the oil (looks like dirty mayonnaise on the underside of the oil cap)
And if you see any of these get it checked out as soon as possible.