The oil and filter should have been changed along with new head gasket. Was the level correct when you had the car back?
Some cars can `use` a litre (or more)of oil per 1000 miles. weekly checks are always recommended.
I would top it up to the recommended level and check level daily. If in doubt take it back to the guys that did the head work.
If the car overheated when the head gasket went it may have caused other damage.
im gona ring them in the morning i stil owe them money for work thats been doone and they anit getting it until its sorted. something anit right with it as its never used that much oil cause i had head done last year and this year..