I would like to mask out a design on a mold prior to spraying gelcoat.

It won't affect the release properties at all, but you will want to be aware the masking tape will not stick to a surface prepared with easy-lease. It will hold lightly, but it will come off very very easily. The air pressure from spraying would possibly be enough to blow it off, depending on your surface porosity. What will likely happen is the edges of your tape will lift from the surface allowing the coating to bleed underneath and ruin your lines.