In order to allow clients on your network to connect via your gateway, you will need to enable PPP's network address translation (NAT). In order to use PPP's NAT functions, add the following lines to /etc/rc.conf:
ppp_enable="YES" ppp_mode="auto" ppp_nat="YES" ppp_profile="your_profile"
Note: Take care to change your_profile to the name of your own dialup profile. The profile name should match the name of the dialup connection in your /etc/ppp/ppp.conf file.