I just checked to make sure but the prices on the Belgium Intercity trains are the same price if booked ahead or for today, so you do not need to do anything ahead of time.
I would download the SNCB app and just book the next train once you get thru Immigration and Customs. You can also go to a ticket window and purchase. If you are nervous, have typed out the train station which is "Gent St. Pieters".
https://www.belgiantrain.be/en
I have not done this train journey but it will be very easy and straightforward. I came in from Paris to Gent via Brussels and even changing trains in Brussels Midi station is not difficult. There are plenty of people who speak tourist English who can help you if needed.
I do all of my RS tours as a solo including this one. I had a wonderful time! It is a great tour and I highly recommend it. I might have mentioned this before but if you can, go in at least 1-2 days ahead of the tour start just to cover any travel hiccups that might happen. I love the Hotel Harmony if that is where you will stay. Awesome breakfast buffet even for a non-breakfast eater like myself. Who can resist chocolates for breakfast?
editing to add: I just looked back on your posting profile and see you enjoy France...so Paris is a wonderful pre-quel to this tour, hahaha. VERY easy to take the Thalys train from Paris Gare du Nord to Brussels and then the intercity out to Ghent/Gent. IF you do that, book on the Belgium train site as you will get the trip to Gent included on the itinerary. Here's a link to my TR from April 2019 - so pre-covid.
https://community.ricksteves.com/travel-forum/tours/trip-report-rs-belgium-holland-4-6-4-16-2019-plus-time-in-paris