I solved this by changing the method of connection to the server from TLS to SSL and the port to 465 instead of 587.
DId you ever get this resolved? I’m running into the same issue.
I am having this exact problem with any form submissions that send emails as of yesterday. Like you I’m experiencing this with internal form submission, like adding a user, or contact forms. Again, anything that emails. So users stared at a blank screen, and were getting a 504 error until we created a temporary workaround by editing the htaccess file which lengthened the timeout period. This isn’t a good long term solution obviously. I timed up to a minute wait for the submissions to go through, and of course users will probably not wait that long or believe that it didn’t work.
I think something happened yesterday that is causing this.. perhaps Gmail updated something that broke the plugin?
