If I understand your question correctly you are using Maximo Service Desk to manage service requests for a particular software product which also uses Git for its source code version control system and you are looking to relate work items in Git (e.g. issues or commits) to service requests in Maximo.
That being the case, there is no Out of the Box functionality to integrate Maximo and Git. It would potentially be feasible to achieve this using Maximo customisations (e.g. Jython automation scripts) and the Github API but it would not be a small task to do this.
One manual workaround would be to post the url to the work item in Git in the details of a new Work Log associated with the Service Request.
You could also request IBM include this in a future Maximo release.