I have an adjustable gas plug and would recommend getting one if your ammo choice are limited.
There are two types available,
The Mc Cann Industries uses a lock screw that comes with several different set screws, each screw tailored to bleed off a specific amount of gas.
The Schuster lock screw has a single set screw that is backed off, then adjusted forward until the rifle just begins to cycle with the ammunition you are using.
If you are looking to tailor to one specific load, I would recommend the Schuster plug, which can be set, then Loc-Tited in place for that single load, the screw won't back out if you do this, causing the gun to malfunction.
The Mc Cann is a better choice if you think you may be using a number of different ammo choices.
The proper set screw for a specific load can be determined and then marked with paint or engraving, identifying it for that specific load.
One thing, I don't recommend using bullets heavier than 168 grains in the M1 Garand, especially using commercial loadings, even with the adjustable plugs.
Some of the heavier loads are very heaqvy loaded and may cause damage to your rifle, even with an adjustable plug in place.
Avoid any commercial loads that are advertised as "light magnum", "Extra Power" or any such indicators the stuff is hot loaded. HTH
Here are links to both products.
http://www.brownells.com/.aspx/pid=20456/psize=24/Product/ADJUSTABLE-GAS-SYSTEM
http://www.brownells.com/.aspx/pid=1538/Product/M1-GARAND-ADJUSTABLE-GAS-SYSTEM