Hi Brian and welcome to WetCanvas and Mixed Media. I don't paint watercolor but I would imagine that it is the gessoe that makes it not adhere the WC. I would try to put acrylic paint over it or just matte white house paint, that would make it a little more porous and let it accept the WC. If that would not work, since you have already bought them I would paste some kind of watercolor paper on it with a gel medium or just crafts glue.
I hope this is of help and I am sorry that these are just suggestions, but watercolor is a medium I have never used.