I booked Ouigo train journey from Paris to Nimes Pont du Gard, (arriving there 14:37 on 26 May) but later realised I need to go to Nimes centre instead. There seems to be a TER train to Nimes centre at 16:00 which is rather annoying as it means waiting for over an hour. Can anyone tell me please: Will I need to buy a further ticket or is it included in my Paris to Nimes Pont du Gard? I saw somewhere that it is included but now I can't find it.

Instead of waiting an hour at PdG you may be better off rethinking the entire journey from Paris and optimizing your whole trip. That may mean a different Ouigo to PdG, or connecting somewhere like Lyon...

No, it's not included. Only costs a few euros, www.sncf-connect.com. But why not just take a taxi?

It never would be included as it's a later stop. There is a bus earlier around 2.57, but there are plenty of trains you could have picked instead from Paris to Nimes. I'd change tickets from paris.