If you haven't heard, Gutenberg is coming to WordPress. If it's still on schedule, that could be next month; April! You should start testing and planning for it now.
You can easily test Gutenberg by adding it to your site. It's available in the plugin directory as a stand alone plugin. Install it, activate it, and then create a new post. See what happens.
If you use a lot of custom fields, then you may have some things to work through, but then again, you may not. All my custom fields still show at the bottom of the post, but I don't depend on them for layout or design so I maybe got off easy.
Everything Might Be Fine
Chances are that Gutenberg may work just fine. The perceived issues may be bigger than the actual issues. I didn't have to change any code or do anything special to make Gutenberg work on my site.
In testing a bunch of random Gutenberg features in this post, which is why this post seems a bit chaotic, they just worked. This is impressive as this I use a third party theme that I started hacking as they abandoned it years ago and Gutenberg still works fine.
If your site explodes (overreaction) due to Gutenberg, then chances are your developers were terrible, or you have something super custom. Now is a great time to start planning on how to make everything Gutenberg compatible or to figure out your backup plan; like using the Classic Editor plugin to disable Gutenberg.
In the end, chances are, you may just need some style updates. However, you won't know until you test, so start testing!
With so much noise online about the WordPress 5.0 release, what you need to know is: Ultimately Gutenberg is a planning issue, not a technical onevia @crowdfavorite
What are you waiting for? Start testing your site today. If there is something to worry about, you have time to make a plan. If it works fine, then you can stop worrying. Just don't wait to the last minute, or say you were surprised when Gutenberg launches. You know it's coming, so do something about it.
In no way am I saying that you should start using Gutenberg full time today or that you should test in production. Gutenberg is still evolving and changing so there is risk. Backup everything, test in a development environment, and make smart choices.