Find and garbage collect all Git repositories in path `php git-gc.php /path/to/walk`.

git-gc.php

<?php
declare( strict_types=1 );

$path = $argv[1];

$iterator = new RecursiveIteratorIterator(
	new RecursiveDirectoryIterator( $path ),
	RecursiveIteratorIterator::SELF_FIRST
);

/** @var SplFileInfo $file */
foreach ( $iterator as $file ) {
	if ( $file->isDir() && '.git' === $file->getBasename() ) {
		echo $file->getPath() . "\n";
		exec( "cd {$file->getPath()} && git gc --aggressive" );
	}
}