I would like to implement a custom DM interface through Cadence's GDM.
From the limited document, I figure out that GDM requires a shared object library to be built and cadence will use that library. Where can I find more info regarding how to build this shared object library?
How to do a GDM integration is not documented for general public consumption, because it is something that is rarely done and not a trivial task (I know of less than a handful of cases done by people other than Cadence Connections Partners). So it's normally done as a service - providing the integrator's guide, an example integration, and some consultancy.
For more details you should contact your account manager or talk to Cadence Customer Support to investigate further.
There are four connections partners that provide such tools, so most customers end up using one of these (listed in alphabetical order):
These integrate various different underlying version control/configuration management tools (some proprietary, some "open", some supplied by other companies).
For most people, the cost of developing and maintaining such an interface when there are dedicate companies to do this is not worthwhile.