[BigAls87Z28]

Correct. The CTS and SRX and probably the STS all have an odd job GM Group size 100 battery that no one in the aftermarket seems to make.
The 100 group is as wide and long as a group 75 which is a standard battery for GM cars, but the 100 is shorter so it fits in the space under the cowl. 75 is about 1/2 an inch too tall.
Duh Delco makes one, thats the battery thats coming out of the car. And most likely with the neg terminal coming out of the battery as per usual of all side terminal Delco overpriced garbage.

Trunk or in car mounted batteries have vent tubes to move the gases to the outside.

A lot of the new GM cars have switched to top mounted terminals. Other GM cars that no one makes batteries for would include the Cobalt/Ion/HHR, last gen Grand Prix with top terminal, new GM trucks and SUV's...pretty much every GM car from about 05-06 foward.