Thank you so much for you help. It is still not working. This is what I am doing:
My build.gradle looks like this: https://pastebin.com/LUVFJvhy (The part you suggested is at the bottom)
And when I open a command prompt and navigate to my forge folder, I type:
gradlew runClient -PappArgs="['--username DamianPilot382']"
When I run it, it gives me no errors, yet the name is still "player###." At the beginning of the launch, I see this in the log:
[21:51:10] [main/INFO] [GradleStart]: Extra: [--username DamianPilot382]
[21:51:10] [main/INFO] [GradleStart]: Running with arguments: [--userProperties, {}, --assetsDir, C:/Users/Pilot/.gradle/caches/minecraft/assets, --assetIndex, 1.12, --accessToken{REDACTED}, --version, 1.12.2, --tweakClass, net.minecraftforge.fml.common.launcher.FMLTweaker, --username DamianPilot382, --tweakClass, net.minecraftforge.gradle.tweakers.CoremodTweaker]
Again, thank you for your help!
P.S. I tried adding a coma in between "--username" and "DamianPilot382" and it didn't work.