<alertbox> WARNING: This document may contain errors which may result in damage to your router. USE AT YOUR OWN RISK. </alertbox>
The Linksys WRT54G* routers were the first routers to work with 3rd Party firmware. However, be aware that newer WRT54G routers do NOT work with OpenWrt or Gargoyle – they have only 2.0MB of Flash, unlike their predecessors which had 4.0MB which is sufficient. If you want to use a Linksys WRT54G* router with Gargoyle, hunt down one of the old WRT54G units, v1-4 or obtain a Linksys WRT54GL or WRTSL54GS.
These routers have a broadcom wireless chip, so you will want to use the brcm-2.4 images. There are several *.bin images for different models and a generic *.trx image. The *.bin images are identical to the generic *.trx image, except they have a model-specific header that allows for easy installation. You can select the appropriate *.bin file for your router and upload it with the original firmware, and it should work ok.
If you upgrade using the Gargoyle web interface to a new version of Gargoyle or to a different third-party firmware use the .trx file. If you are installing from another third-party firmware such as DD-WRT or Tomato, you should also use the .trx file. The .bin files are only appropriate when installing from the stock firmware.
It is also possible to upload the image to the router via tftp. As the router boots there will be a few seconds when it will accept tftp connections on ip 192.168.1.1 (regardless of what you have the ip of the router set to). Follow these instructions to install the image via tftp. Transferring a new firmware image via tftp is also the best way to “de-brick” a router that is malfunctioning.