Congratulations!!! After spending months trying to find firmware that would enable me to use my wzr-hp-g300nh as a wireless repeater I call off the search and I can confirm: IT WORKS! and it is stable. OpenWRT and DD-WRT did not work for me (connections froze after a while), but Gargoyle 1.3.8 does.
Installation is pretty easy: Select "Wireless Bridge/Repeater", then change IP and gateway accordingly and select "connect via client bridge" with "repeater enabled". Fill out the wireless section accordingly and there you go: you have a wireless repeater. Currently I am using different SSIDs for the repeater and the repeated network, so I have to check whether it works with the same SSID, too.
One option I had to manually adjust in /etc/config/wireless was to hide the SSID broadcast. I had to append
option 'hidden' 1
in the appropriate section, because there is no option for this in the gui (yet).
The only downside I see is that I can only connect with 65 MB/s, but since I only wanted to extent my internet connection with a strong signal to other parts of the house it is no problem for me.
I am happy, keep up the good work....