Our development team is aware of the interest in loading scripts in the footer. We did make some changes, still in experimental phase, and they seem to be working fine in our testing but we still need feedback from users. If you want to give it a try just copy/paste the following snippet: https://d.pr/n/vK8iRS
PHP code usually goes in your theme functions.php file or a custom functionality plugin. Check the following link for more details about where to place that code: https://docs.gravityforms.com/where-do-i-put-this-code/#php
To prevent losing your snippet in case of a theme update, be sure you are using a child theme.
IMPORTANT : Gravity Forms 2.5 (which is in beta release now) is planned to have full support for this without using the above snippets. In fact you will need to remove the snippets before upgrading to 2.5 when available. Please keep that in mind if you decide to use this snippet with Gravity Forms 2.4.20 or anything before Gravity Forms 2.5.
If you have any other questions, please let us know.