Hope the experts could weigh in. My plumber told me that the vent from the gas dryer to the outside cannot by code dip at any point. That is it can only go straight or up. However, having looked at what I believe is the relevant section of the code http://www.nyc.gov/html/dob/downloads/pdf/mechanical_code.pdf, section 504 I could find no mention of this. The installation manual for the dryer did not mention this either. I do understand that warm air goes up etc etc. The issue a question is a dip of a few vertical inches. What's the right answer according to NYC code?

Don't know about the code, but good practice is no dips, because moisture can condense in the dip.

The code does limit the duct to 25 feet, and each 90° bend counts as five feet. A dip would use up 20 feet of the 25, so maybe that's what the plumber meant.

This is basically a tiny furnace chimney -- as such, don't mess around with it; figure out how to pitch it properly so that it doesn't have to dip. Also, the mechanical code link, while helpful, is not the sole source of rules on this -- there may be official code interpretations, industry rules, insurance requirements, etc. that all get condensed into the phrase "required by code" when your plumber talks to you.

Also don't know about the code, but more so than moisture accumulation lint will accumulate in the dip. Accumulated lint heated by hot air = fire hazard. Ask Robert DiNiro.