public function init()
{
    // get username from url
    $username = $this->property('username'); //matt

    // pass username to scope
    Gallery::applyUser($username)->get();
}

// return results that match username
public function scopeApplyUser($query, $username)
{
    return $query->where('username', $username);
}

Missing argument 2 for MyVendorGalleryModelsGallery::scopeApplyUser()