Cannot access the internet using a default gateway virtual server

You must disable address and port translation on your Virtual Server. These are under the "Advanced" configuration. Without that, all traffic will be forwarded to the pool member using it's IP address as the destination. Moreover, assuming you want to forward more than TCP traffic, you must change the Protocol to "*All Protocols". You almost certainly want to limit the VS to just your internal VLAN (change "VLAN and Tunnel Traffic" to "Enabled on..." and move the internal VLAN to the "Selected" box).

Incidentally, this can be achieved a different way. If you set the BIG-IP default route to 172.16.100.17, you could then use a "Forwarding (IP)" Virtual Server type, which uses the BIG-IP route table to forward traffic.