How to custom page template for the Gutenberg editor

Its looks like you are little familiar with WordPress. No problem.
Yes, its true it does not have any places to edit front-end segments or blocks. That’s because you have to create them in backend.
We can use ACF to create the backend meta boxes quickly. If you wanna do this manually you can check the guide for that.
For simplicity let’s answer your question:

1. go to page / post edit.
2. click on gear icon to open the side panel then choose “code editor” from the menu.
   It will provide you with html editor. you can simply write all your html code here.
   To create a page template:
3. create a file in server > your theme directory
4. Add the page template comment inside that.
5. Check in the back end side panel in page editor. You can now find a drop-down listed with your page template.

Also it has an in built html editor widget to add in to your block editor.