substitute

The white garlic sauce is apparently not too hard to make. There is a greek variant called skordalia that has a thickener like potato or breadcrumbs in it, and a lebanese variant called toum that has egg whites sometimes. Or just garlic, olive oil, lemon, and salt.

http://greekfood.about.com/od/syrupssauces/r/skordalia.htm

http://www.lebaneserecipes.com/Toum.htm

There are variants with different oils and no mayo.