Hi @CrucialNic,
just found this post and thought I add my thoughts here. I totally understand what you are after working with WP over a decade as well and using ACF and custom fields as well as layout builders at the moment Elementor or we could even add WP5 Blocks in that mix? ...
Unfortunately Grav doesn't come with a plugin like that. It might be a limitation of the frameworks possibilities.. or maybe someone could build it but it might be a massive task for which revenue? I just think the market is too small for someone to jump it...
Anyhow - you are able with Grav to create custom fields. If you used Wordpress and ACF or you know about the WP custom fields - then Grav comes with a similar counterpart. This allows you to break-out from you standard Markdown editor per page and build your custom fields and layouts really.
Some of theme themes, I am eg. using Quark as a base theme to start out, also come with built in CSS frameworks such as Spectre which come with a ton of features including Grid (similiar to Bootstrap Grids). This can help to build your responsive columns within the standard markdown editor.
Now I know this is far fetched from a layout builder but better than nothing - at least it allows you to manage some aspects of the front-end layout visually....
Hope this might be of some little help.
Sam