Honestly I do not think that this problem is caused by my plugin. I’m not able to reproduce this behavior.
I have already tested all my plugins with Gutenberg and also with WP 5.0.
Just to be sure, I set up a new testing site today. WordPress version 4.9.8, Gutenberg version 4.5.1 and smart User Slug Hider version 2. No other plugins. Creating posts and pages works fine.
Your problem probably is caused in combination with another plugin or your theme. Please follow the steps in this checklist https://petersplugins.com/please-follow-these-simple-steps-if-you-have-any-problems-with-one-of-my-plugins/ and inform me about the result.
If you find out the reason for this problem, I’ll try to fix it.
Regards,
Peter