Apparently GOG no longer accepts bank transfers through PayPal. And apparently bank transfers are now called eCheques. Forgive me if this is old news, I haven't bought a game in a while.

Try the instant transfer. You may have inadvertently clicked the eCheck option. eChecks have been around a while. Whats good about eChecks, is that the recipient gets absolutely no information about your banking account. They don't even know what bank you use. But eChecks take 3-5 days to process. So I can see why retailers of digital products won't accept them. Try the instant transfer.