The best approach is to select a Router used successfully by many other Gargoyle users. Other Routers may work, but you will need to check things out more carefully.
Check the exact version of your hardware carefully.
Use factory.bin to install Gargoyle over the original manufacturer's firmware
Use sysupgrade.bin to upgrade an existing Gargoyle/OpenWrt installation
It is important to read the instructions carefully to enjoy a trouble-free installation. If you do get into trouble then the recovery instructions will probably save you. If you get homesick then you may wish to revert.
Once you get started with Gargoyle, it is easy to connect your devices to the Internet; and to monitor realtime bandwidth for each device. Setting up quotas to manage your bandwidth is a little more involved, but well worth the effort. Turning on the default Quality of Service management will prioritise your bandwidth when your Internet connection is under full load. And if you get stuck then try the forum.
Frequently Asked Questions about the Gargoyle Router Project.
Infomation found here may be helpful to Gargoyle developers.