I tried The openwrt and Gargoyle firmware with my new WZRG300NH? but mine has A2 on the back wich I think is an updated model.
I used tftp with this app (seemed to work more reliably) http://kin.klever.net/pumpkin? in windows 7 and the router would just sit thier unable to be accessed until I tftped the normal friendly firmware back to it. Each time I gave the router 10 min for the firmware to install but it does not work.
So These firmwares, The latest gargoyle build "gargoyle_1.3.12-ar71xx-wzr-hp-g300nh-squashfs-tftp.bin"? and openwrt "openwrt-ar71xx-wzr-hp-g300nh-jffs2-tftp.bin" DO NOT work with this new model according to my testing.
I hope this saves you wasting your time as I see no mention of this anywhere, that said the latest friendly firmware tftped onto router works great