use a clear silicone to seal the edges. This will prevent any water seepage from getting under the sink. Depending on how big of gap it really is (as it is hard to tell from the picture) a sink replacement may be preferred. Was this a final sale item or a "special order"- if it was a special order, you may return as a manufacture defect as there was something that went wrong with the mold, this is common if you purchased your sink at a retail giant as a lot of them purchase what we call in our industry as seconds.... meaning not up to industry standards.