I have a gap in my concrete window sill, what do I fill it with
I use Quad Window Door and siding sealant.

You can get in grey.

Mask off the sill so you you do not get it where you do not want it. Fill the gap then take you finger and dip it denatured alcohol and use your finger to smooth it out, do not work it to  much. Pull your tape and your done. 

It will help to do a practice run on some scrap wood or something so you understand how to work with it.