First what are the options in Group Policies that affects the remote access clients? their should be nothing between them, or may be you mean Remote Access Policies. Please clarify this by examples
Second, what type of firewall do u have, if its just a normal firewall with no NAT capabilities, then you will have to open alot of ports, this will depend on what your remote access clients are trying to access, but if the Firewall has some NAT capabilities like ISA server, then this will not be possible.
Another point how does your remote access client connect? through VPN or dialup?