What kind of appliance(s) used to be here?
This place obviously was occupied by a gas dryer. 

Of course, it's impossible to know for sure, and what is obvious is not always true, but nothing else is really likely. 

Venting a gas dryer downward presents no special difficulty, because the exhaust is driven by a powerful fan. Many dryer models offer the choice between rear exit or bottom exit. The vent pipe can be up to about 25 feet long but bends and flexible sections reduce the allowed length. 

The height of the gas pipe is more usual for a stove than for a dryer. Perhaps at one time a stove stood there, then subsequently someone cut the round holes and installed a dryer instead. 

The D-shape of the vent passage is strange, but as Mike Harris and Alaska man indicate, this is probably an accidental result of cutting holes in a finished and occupied structure. 

The goal of a home owner is often different from a professional contractor for the  same job -- the contractor aims to do a sparkling clean, plumb, and finished installation that reflects well on his skills and motivation, while the home owner aims to get something working without spending a small fortune or burning down his house.