At the moment there is no 3rd party firmware available for the WRT54GX and therefore you can't run Tinypeap on this device. This might change, as some firmware developers like Sveasoft have announced, that they are planning to support the WRT54GX in the future.
But honestly I don't expect to see a 3rd party firmware for the WRT54GX in the next few months. Sourcecode has been released for many different Linksys wireless devices. But nevertheless 90% of 3rd party firmware development is still happening only for WRT54G, WRT54GS and NSLU2.