If you wish to sync your GB reusable blocks between sites in your network,you can use the free Broadcast plugin.

Just add the wp_block custom post type to the Broadcast settings and you can broadcast the blocks when you edit the blocks.

The above will enable you to broadcast the blocks and then add them to posts on each child blog.

Using broadcast reusable blocks in broadcast posts

If you broadcast pages / posts that use reusable blocks, you’ll find that the blocks on the child blogs are “broken”. This is because the block IDs do not match up: a reusable block on blog 1 could have the ID 123, but will have the ID 456 on blog 2.

To fix this, you will either have to modify the broadcast page / post manually, or use the Gutenberg Posts add-on that will modify the block reference automatically.