Q: Blown head gasket? 2000 VW Golf

Hello - yes, this bubbles in the coolant reservoir could indicate a blown headgasket. There is a specific test which determines if the bubble in the reservoir contain combustion gases. This would confirm a blown headgasket. Checking for low compression on 2 adjacent cylinders can also indicate a blown head gasket. I recommend a loss of power inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.