I was outside during a torrential downpour of rain.
There was a sudden torrential downpour of rain, and I got soaking wet. I happened to be outside during that time.
ご質問ありがとうございます。
「ゲリラ豪雨の時に外にいた」は英語で「I was outside during a torrential downpour of rain.」と言います。
また、「突然のゲリラ豪雨に遭い、びしょ濡れになりました。その時に自分が外にいた」と言いたいなら、そうすると「There was a sudden torrential downpour of rain, and I got soaking wet. I happened to be outside during that time.」になります。
ご参考になれば幸いです。