I have done the trip from Gent to Brussel many times by train. Gent is served by two stations, the main one is St Pieters, but both stations provide trains to Brussel. The ones from St Pieters tend to be more frequent and faster.
The train from Brussel to Luxembourg is frequent and direct, and is - as mentioned by Philip - not a bullet train. It tends to be operated by slightly older equipment and never exceeds about 160 km/hr (100 mph). The scenery is pretty nice.
Luxembourg operates a very integrated transport system, with trains going down several branches and supplemented by buses going to most cities and towns linked by connecting buses. Most CFL (Luxembourg trains) are modern double deckers which give nice views. The CFL buses are almost all modern air conditioned single decker front entry buses which are closer to coaches than buses.
Luxembourg City is easily walkable and lifts are provided to change levels in several places.
Luxembourg to Paris is a route I never have done. You would take a train from Luxembourg due south to Metz in France which I understand is a nice city. You then get the train into Paris. I haven't looked it up but I think that most high speed TGV Est trains to and from Strasbourg would bypass Metz, but there are certainly trains.