I'm trying to add all my 'args' into runClient in build.gradle but for some reason they don't want to load until I use '/gradlew runClient", I'm using IntelliJ. Is there a way to do this? Since I sometimes find it annoying to every time I make a new project I need to add the arguments I want... its the same for 'jvmArgs' too.

runClient {
    args = ['--username', 'name']
}

I've also tried connecting them into one string... removing the equals sign.... but none helped, IntelliJ just wasn't detecting it.

Running Minecraft through IDEA doesn't use the runClient task, you need to add the arguments to the IDEA run configuration.