Oh, I can see what you mean! GTmetrix isn’t waiting until the site is fully loaded to display the results.
I’m afraid that there’s little we can do to help without knowledge of how GTmetrix loads gathers information from your site. You could contact GTmetrix to report the issue and discuss it further with them:
https://gtmetrix.com/contact.html
Alternatively, have you considered using another tool to analyse your site? It looks like Google’s PageSpeed tool waits for your site to fully load and will give your some more accurate results around page speed:
https://developers.google.com/speed/pagespeed/insights/