If you've connected the Linksys router's RJ45 ethernet WAN port to one of the Gigaset's LAN ports, it (the Linksys) will behave as though it is connected directly to the ISP's own router. Therefore any port forwarding or other configuration should only need to be done in the Linksys.
The one important requirement when using a sub-router in this way is that both routers operate in a different subnet (IP address range). The third octet (number) of each router's IP address must be different, e.g. 192.168.0.1 and 192.168.1.1
The sub-router's WAN IP address would normally be an internet address allocated by the ISP, but in your case it will be a LAN IP address allocated by the main router. DHCP must remain enabled in both routers so that the main router allocates an IP address to the sub-router and the sub-router allocates addresses to connected computers.