Yannick Lefebvre

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site. The number of times that these can load is configurable. They can load content from external sites or custom HTML code into the dialog. This plugin can be used to invite people to register to a newsletter, respond to a survey, or simply welcome come to a site upon their first visit.
You can try it out in a temporary copy of WordPress <a href="https://demo.tastewp.com/modal-dialog" rel="nofollow ugc">here</a>.

Modal Dialog

The others either didn't work or caused massive JavaScript problems, breaking other plugins.

This plugin worked the best.

Worked better than the others

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site.

Alex (Shurf) Frenkel

A widget to add Gravity Form in dialog popup, has an option to add a delay, a position, and an introduction page.
From version 0.3 can be opted in/out to work ont he homepage. 
From version 0.5 you can use a button to open the popup. 
From version 0.6, added an option to show the popup only ones in X views (random generated) – Requested by .
<h4>Under the hood:</h4>
<ul>
<li>Built on Gravity Form 1.5.2, and uses it for working.</li>
<li>jQuery UI added from WordPress itself</li>
<li>Uses jQueryUI CSS</li>
</ul>
Enjoy using Gravity Form Popup Widget? Please consider <a href="http://alex.frenkel-online.com/donate/" rel="nofollow ugc">making a small donation</a> to support the software’s continued development.

Gravity Forms Popup Widget

A widget to add Gravity Form in dialog popup, has an option to add a delay, a position, and an introduction page.

Modal Dialog vs Gravity Forms Popup Widget