The GA travel card is primarily for Swiss residents.
You want to look at the passes available to tourists. Most get the Swiss travel pass or the half fare card. Sometimes combined with a regional pass.
If you’re just buying single ride tickets there is no need to buy ahead of time unless you are getting a super saver ticket which is for a specific train and time.
Regular tickets are good for any train and time. For example, if you buy a ticket for tomorrow for Zurich to Bern, you can take any train you want and sit anywhere you want in the appropriate class—first or second.