Lemon liqueur has gas?
The gas is likely due to fermentation of the sugars by wild yeasts which are present almost everywhere.  This is the standard method for production of alcoholic beverages, although specific yeast strains are used.  It is obvious that there was insufficient vodka to prevent yeast growth.

As far as consuming; there is no way outside of a lab to determine the yeasts and their products, as well as any bacteria - and their products - that may have participated.  As such, I would hesitate to consume it.