Hello,

I have deactivated all the plugins and switched to the parent theme, and no gap under the footer: https://gyazo.com/e152e95ac0bb42b3b05d66f6d01cdd86

Taking into consideration that it is a staging site I leave it as it is now for you to check.

Please activate the plugins one by one checking the footer in incognito and then activate the child theme to detect what causes the problem.

Best Regards