Hi again, Lennard.
Finally I’ve done it. From firmware version 3.022 is available a menu option called “VPN Policies” under VPN menu. There you can enable that traffic to an IP, IP range, MAC address or domain (you can also combine more than one using ‘|’ separator) use the VPN connection. Also you can do it inverse: you can enable that traffic to internet uses the VPN except to an IP, IP range, MAC or domain.
As you said, you wanted to use VPN for your guest wifi. Since default guest wifi network in Gl-B1300 uses 192.168.9.1, you can set that just the range 192.168.9.0/24 uses VPN.
Please take into account that this option is only available since 3.022 version so you will need to upgrade the Gl-B1300 firmware before next steps.
Here you can find more information
Let me know if you have any problem or doubt.