Connecting FreePBX with external Gateway


I’m new to FreePBX and IP Telephony and I’m trying set up outbound calls over an external gateway (Telsey GADAM 2+).
The gateway has two phone ports (fxs), two BRI ports (This should be my connection to the outside world), one ADSL IN port and four eth ports.

I’m not quit sure where to start with the configuration for this outbound telephony part.
My local SIP telephony works fine and was easy to set up, but now I’m stuck.