It's an odd combo, bc Xyrem is to make you sleep (in the right dose) and modafanil is amphetamine-ish. I have never taken modafinil "recreationally." I need it to function. I take 200mg twice a day. I used Ghb recreationally during the day with it, for probably 2 months or so.
I never had a problem per se, but I would caution you to be very careful with the G. If you start using it daily, you develop a tolerance very quickly...and G is a unique craving. It's not like H, but it digs in pretty good and quitting it can be very difficult. I've seen some people who seemingly had no "addiction" issues, until they started dosing G during the day. I can confirm that it's not a fun drug to break loose from.