META-INF file is used for ATs, doesn't have to do anything with Maven.
If you're using IntelliJ just add the .jar files to the libs folder and it will work, you will probably have to refresh gradle by using that button in gradle module,
If you're using Eclipse you have to add something to your gradle.build file for it to work
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar']) // For all .jar files in folder
compile files('libs/file_name.jar') // For one .jar file in folder
}
The mod will be ran with the client too, adding it again to the mods folder will just cause a duplicate mods error.
NOTE: Make sure to use deobfuscated jar files.