That does sound like the head gasket.
They're pretty robust normally and not particularly weak.. but they can fail from time to time.
Gaskets don't like "just" water as coolant. They look like plastic/fibre .. but they have one or more steel mesh inner layers. The coolant gets inside the layers, where it will corrode the steel mesh if it doesn't have any corrosion inhibiter (i.e. "anti-freeze") in it. That causes the gasket to lose strength and fail. Always keep the coolant at the proper 50% concentration.
Yours sounds like a straightforward failure at the fire ring.. just fire, pressure, age and a bit of vibration and harshness may have split part of the gasket enough to cause seepage - when the engine is hot, the coolant is under pressure, so it will escape if it can.
When you have the head off, also change the cam-belt, balance belt, tensioner and pulleys so that you won't have to touch those again for another 3 years/36k... unless the water pump or variator start playing up.
The water pump is on the left of the engine as you look at it, driven by the cam-belt. You could
change it if it's original and your beast has more than 100k on the clock, otherwise they're pretty tough.
If the beast sounds like a diesel when you start it (or clatters) then it may be worth also changing the variator since this is also a cambelt-off job to change if you have to do it later.