Well,the right one is 'αρκετη βενζινη',because we want to show the quantity.If you were saying 'αρκετη η βενζινη',that would be wrong,because 'βενζινη' in this phrase is uncountable. Just keep in your mind that next to words of quantity(λιγο,πολυ,καθολου,αρκετο,etc),the subject is without the article(ο,η,το).