Você está correto em dizer que após preposição usa-se o verbo no gerúndio, porém aqui não se trata disso, e sim de uma expressão idiomática fixa: be about to do somethingbe about to do something
: if someone is about to do something, or if something is about to happen, they will do it or it will happen very soon:
We were just about to leave when Jerry arrived.
Work was about to start on a new factory building.
Hurry up! The ceremony is about to begin!
Do you have a minute? "I'm sorry, I'm just about to leave."http://www.ldoceonline.com/dictionary/about_3