VPN Tracker is the leading Mac VPN client and compatible with almost all IPSec VPN gateways (Try VPN Tracker for free). Please refer to the following table to find out if the VPN Tracker team has already successfully tested VPN Tracker with your Linux VPN gateway. If configuration guides are available for your Linux VPN gateway, you'll find them here.
In case your Linux IPSec VPN gateway is not in the list, there's a good chance that your Linux VPN appliance will still work with VPN Tracker. The VPN Tracker manual has general configuration instructions.