@PhilippGerber-6516 Thank you for your questions.
First you can use your VPN Gateway for both connections: BGP and Non-BGP. https://learn.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-vpn-faq#can-i-mix-bgp-with-non-bgp-connections-for-my-azure-vpn-gateways
Now, If you use BGP for a connection, leave the Address space field empty for the corresponding local network gateway resource. Azure VPN Gateway adds a host route internally to the on-premises BGP peer IP over the IPsec tunnel.