Book train Paris to Brussels-Zuid and then take change to the trains for Gent, there are dozens of trains daily so no need to worry about sked.

Do not book in the US on Rail-Europe, more expensive and most tickets non-refundable.
Wait till the day of travel (From Bussels to Gent) and just go to the station, We were able to get a special fare for two pax travelling together.
Fares etc on link below:

http://www.thalys.com/

mike