According to the OpenWRT R6220 device page, 91MB of the available flash is either unused or reserved, probably to be compatible with easy reversion to OEM firmware.
See the Software mods section of that page about possibly being able to use the 60MB reserved partition. I expect that you would have to compile your own firmware as the required kmod-mtd-rw package is not built in any normal Gargoyle build and it's highly unlikely you can use any OpenWRT kmod package (you might get lucky with the 24.10.4 OpenWrt package on the 24.10.4 based build from Lantis but the odds are against you as Linux kernel builds are very finicky about everything matching).