Not sure if the issue is on the OpenWRT side or Gargoyle, but it appears that it thinks that the router has 64Mb instead of 128Mb. I would think this is a kernel issue? I also have a WD N600 and on that one Gargoyle sees full 128Mb.
This would be an upstream issue.
Please check the behaviour on Openwrt
https://lantisproject.com/downloads/gargoylebuilds for the latest releases
Please be respectful when posting. I do this in my free time on a volunteer basis.