Example Gutenberg block with server-side rendering. Gutenberg edit() block creates interface. Gutenberg saves settings automatically, the PHP function passed as `render_callback` to `register_block_type` is used to create HTML for front-end rendering of block.

serve-side-block.js

const { __ } = wp.i18n;
const { registerBlockType } = wp.blocks;
const el = wp.element.createElement;


registerBlockType( 'hiRoy/serverSide', {
    title: __( 'Server Side Block', 'text-domain' ),
    icon: 'networking',
    category: 'common',
    attributes: {
        images : {
            default: [],
            type:   'array',

        }
    },
    edit({attributes, setAttributes, className, focus, id}) {
        //Put a user interface here.

    },
    save({attributes, className}) {
        //gutenberg will save attributes we can use in server-side callback
       return null;
    },
} );

server-side-block.php

 <?php
 register_block_type('hiRoy/serverSide', array(
            'render_callback' => 'hi_roy_render_callback',
            'attributes' => array(
                'images' => array(
                    'type' => 'array'
                )
            )
        )
    );

    function hi_roy_render_callback( $attributes ){
        $images = $attributes[ 'images' ];
        return '<div><!-- put image gallery here--></div>';
    }