A CRJ200 flight crew experienced LANDING GEAR DISAGREE warning after takeoff. They ran the checklist; declared an emergency and returned to their departure airport; where they found the left main landing gear was still pinned.

Narrative

As a reserve pilot; I received a call from Scheduling at XA33 for an XC00 report time. On my way to the airport; I received a call from Dispatch asking to depart on time if at all possible. The reason for this call was that the crew for this flight was not scheduled for the usual 45 minutes from our show time to departure time; but only 30 minutes. At my arrival at the airplane; the First Officer was already present and started setting up the aircraft and did the walkaround for the preflight duties. The flight pushed off the gate 3 minutes before scheduled departure time. Just after takeoff; we received the Master Warning for Landing Gear Disagreement. We ran the required checklist; declared an emergency; and returned to ZZZ. Upon arrival at the gate; we discovered that the left main landing gear was still pinned. The fact that the crew did not receive the proper callout time/report time to operate this flight; certainly was a factor in this incident. Also; the call from Dispatch put more pressure on us to depart this flight on time.