Hello Marco,
Are you trying to do this on a ASA?
If so, you can forcefully give a default gateway for the DHCP clients connecting to any port. the sample configs are as follows:
hostname(config)# dhcpd address 192.168.3.5-192.168.3.250 DMZ1
hostname(config)# dhcpd dns 209.165.201.2
hostname(config)# dhcpd wins 209.165.201.5
hostname(config)# dhcpd lease 300
hostname(config)# dhcpd option 3 ip 192.168.3.1
hostname(config)# dhcpd domain your company.com
hostname(config)# dhcpd enable DMZ1
I have considered that the clients terminate on DMZ1 interface. you can alter the configs to your setup ....
Try this and let us know... rate replies if found useful..
Raj