De or het lunchmeeting?

The correct Dutch article is:

de lunchmeeting

Rule Words ending in -ing: almost always de. See the rule

How to use lunchmeeting

the lunchmeetingde lunchmeeting
this lunchmeetingdeze lunchmeeting
that lunchmeetingdie lunchmeeting
our lunchmeetingonze lunchmeeting
your lunchmeetingjouw lunchmeeting
every lunchmeetingelke lunchmeeting
which lunchmeeting?welke lunchmeeting?
a big lunchmeeting / the big lunchmeetingeen grote lunchmeeting, de grote lunchmeeting