How to use a template file to show all post having same metakey

You might give it a shot this way:

add_filter( 'template_include', 'check_post_meta_template_include' );
function check_post_meta_template_include( $template ) {
    if ( !is_single() ) {
        return $template;
    }
    global $post;
    if ( $post->post_type != 'post' ) { // Proceed only if post type = post
        return $template;
    }
    $category_meta = get_post_meta( $post->ID, 'Category', true );
    if ( $category_meta != 'গল্প' ) {
        return $template;
    }
    // Use locate_template() here.
}

Reference: locate_template