I don't know what type of Garand Match you are going to. My club holds two types 1) one where only people who use Garands are eligible for prizes, or 2) one that follows the CMP rules for a Garand Match. That is, a rack grade rifle, 200 yards, and a course sequence that is fired "backwards".
The Camp Perry Garand CMP type match is fired with issued ammo. Maybe yours allows byoc.
I will assume a CMP type match.
Bring a spotting scope.
Bring a Mat. If you don't have one, roll of carpet, canvas tarp, something thick to lay on.
Put a sling on your rifle.
Bring extra clips.
Got a Single Loading device SLED?
Bring a shooting stool.
Don't forget ear and eye protection.
When it gets to be summer time, don't forget a towel to wipe your face.
Do you have a shooting coat, or just a coat? I use to shoot with the canvas army field jacket and a welder's glove on the support hand. Now I have all the equipment.
Come with a plan. You don't get an alibi. So if your rifle pukes, jams, clips don't feed in rapid fire, have a plan.