Yes you need a security interface/bypass. It is possible the base line models don't but as far as I know ALL GM's since ODBII have been incorporated have a factory security system. (ODBII was added in some vehicles as far back as '95)
I have seen two different types of bypasses, electronic ones that talk over the data bus, and modules that actually physically bypass the alarm. The electronic ones are somewhat more secure because it would be much harder for a thief to start the vehicle with an electronic bypass as opposed to a mechanical one.
The mechanical ones I mention are ones that bypass the security without touching the databus or talking to the vehicle. The electronic ones bypass the security by broadcasting a "Everything is ok" message on the databus.
Also electronic ones generally require programming from a dealer and most dealers won't touch them unless they installed it too.
Also if you use an electronic bypass in some cases they allow many other features and make your wiring much less difficult because they talk over the databus for things like the courtesy lights, the door locks etc.