Most European countries import materially all the oil they consume, and the expenditure constitutes a substantial share of GDP. For example, Greece's oil import bill was about 4.7% of GDP in 2013.
With the collapse of oil prices, the oil import costs also fall, and fall substantially, which manifests as a direct gain to GDP.
The impact in 2015, assuming current oil prices sustain for the balance of the year, is quite notable, from around one percentage point France and Germany, to 1.7 percentage points in Portugal and Greece.
This number excludes multipliers or other positive effects. Rather, this is purely the effect of paying less for oil. It's quite a substantial sum of money.