I suppose with GB you mean "global”. (I first though Great Britain...)
Optional means optional. It's up to you whether you want to make a seat reservation. Some operators will show how busy they think the train will be, e.g. DB (bahn.de) or ÖBB (tickets.oebb.at).
Required means you have to have a reservation to be able to use that train.
For the best ways to book reservations, check this page:
https://www.seat61.com/interrail-and-eurail-reservations.htm
Please note that you don't need to activate your pass to be able to make reservations. Moreover, it's not a good idea to activate a pass in advance. Plans can change, eve et the last minute e.g. because you wake up ill on the planned start date. You can only cancel activation until 23:59 CET before validity starts and after that it's too late and you'll loose travel days.