Painting Clear Gesso Over Acrylics or Spray Paint - is clear gesso really clear?
For your black piece, a slightly better alternative would be black gesso, since clear gesso is slightly cloudy. Liquitex and Golden make it.

For your smaller piece -

If want to draw on your surface and need a lot of tooth, then clear gesso will help. Clear gesso is very toothy, and too rough for some applications.

If you're afraid that your new layer of paint won't stick to the spray paint, then try matte medium, which has a little tooth. Or you could mix matte medium with clear gesso to get the properties you want. Or you could sand the slick parts. 

Your previous layer of acrylic paint should not be dissolved or washed away by the gesso or matte medium, unless it's not fully cured, or you thinned the acrylic paint with too much water. Use THIN coats of gesso or matte medium, and minimal brushing.

It might make glossy areas less glossy, and porous areas more wet looking.

It is slightly opaque, but not noticeably cloudy in two thin layers.