You cannot gate check a bicycle. However, I have done a few out of town Triathlons and have checked it through as baggage; I listed as a non-rev and travelled in uniform. Ordinarily it is $200 for mortals but Delta didn't charge me being an employee. I have a hard bike case that protects the bike; the wheels, pedal, and handle bars have to come off.
Now as crew member; I plan to do a Tri in DC on a layover in Sept. As a crew member, I cannot check it like previously mentioned because there is no reservation. I asked the accomodating folks w/ TSA at LGA. They said just give myself additional time and they will open it up and hand check it so I can get it through as a crew member. I can then drag it through and throw it in the belly much like I would for any crew bag I chose not to keep in the cockpit.