One way or another you have gas in the cooling system.... Whether it's exhaust gases or just atmospheric is not something you can just determine from the bubbling. The easiest way to determine if it's combustion gases is to test the pH of the coolant. Most car parts shops here sell (or can order) pH test kits... it's a simple tester consisting of either litmus paper or a sample vial and pill.
Sure you could have a blown head gasket, or you may just have trapped air in which case topping the radiator off manually and over filling the reservoir will help it purge on its own. This could also be indicative of a small leak.
What type of HG did you use? Was any pitting of the block properly addressed? Was the block inspected for warping?
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