allowedCatList.php

add_filter( 'gform_post_category_args', 'allow_specific_categories', 10, 2 );

function allow_specific_categories( $args, $field ) {
    // add allowed category IDs
    $allowed_categories = array( 12, 34, 56 );

    if ( isset( $args['include'] ) && is_array( $args['include'] ) ) {
        $args['include'] = array_merge( $args['include'], $allowed_categories );
    } else {
        $args['include'] = $allowed_categories;
    }

    return $args;
}

blockCatList.php

add_filter( 'gform_post_category_args', 'change_categories', 10, 2 );

function change_categories( $args, $field ) {
	// add your categories IDs (seperate by comma)
    $exclude_categories = '11,32,33';
	
    if ( isset( $args['exclude'] ) && ! empty( $args['exclude'] ) ) {
        $args['exclude'] .= ',' . $exclude_categories;
    } else {
        $args['exclude'] = $exclude_categories;
    }

    return $args;
}