In your given bib entry are some coma missing and there is a wrong }. See the following MWE for the correct entry.
You can use \DeclareFieldFormat{urldate}{visted at #1} to change the printed urldate. Please see that you have to use urldate instead url.
\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@online{gates,
author = {Bill Gates},
title = {Save the world!},
url = {https://www.gatesfoundation.org/de/},
urldate = {2016-07-04},
organization = {Bill and Melinda Gates Foundation},
}
\end{filecontents*}
\documentclass{article}
\usepackage[style=authortitle]{biblatex}
\addbibresource{\jobname.bib}
\DeclareFieldFormat{urldate}{visted at #1}
\begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\footcite{gates} \cite{gates}
\printbibliography
\end{document}
The result:

As mentioned by @moewe in his comment the best solution for your issue is the following:
\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@online{gates,
author = {Bill Gates},
title = {Save the world!},
url = {https://www.gatesfoundation.org/de/},
urldate = {2016-07-04},
organization = {Bill and Melinda Gates Foundation},
}
\end{filecontents*}
\documentclass{article}
\usepackage[style=authortitle]{biblatex}
\addbibresource{\jobname.bib}
\DefineBibliographyStrings{english}{% <==================================
urlseen = {visited at} % <==================================
}
\DeclareFieldFormat{urldate}{\bibstring{urlseen}\space#1} % <============
\begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\footcite{gates} \cite{gates} \citeurl{gates}
\printbibliography
\end{document}