The generic LEDE target for the GL-AR300M works fine.
You will find that LEDE only supports NOR, which is a problem if you have the AR300M version with NAND as well as NOR as NAND is the default boot.
If you start up the uboot UI and it gives you both NAND and NOR reflash options you are good to go.
If it only gives a single NAND option you will have to upgrade uboot first so you can then reflash to NOR.
Finally once you have reflashed the NOR it will still boot into NAND and the switch on the side does not work as a boot NAND/NOR selector.
Instead you have to edit the /etc/rc.local file on BOTH NAND and NOR to change the default boot to NOR. (This can be done in Luci (Advanced in GL firmware, under Startup - custom scripts))
For the modifications, see:
This sounds complicated but actually is quite simple.