A more cost effective solution would be to remove one of the travel lanes on the Key Bridge and then use the space to put protected bike lanes in each direction. While there are three travel lanes in each direction, only two of the lanes in the outbound direction are actually needed as only two lanes turn off of M St. The third outbound lane is added on mid-span and isn't needed to maintain traffic flow. There would need to be some adjustments in the inbound turning lanes, but there's enough space to accommodate.
On each end, CaBi bike share stations can be set up strategically based on commonly used end points, along with way-finding to get users around. This will be much cheaper than building a gondola and much faster to implement.