There is no direct Disney transportation between the theme parks at Downtown Disney (because there was too much abuse in the past), so you'll have to either take a taxi, or do a "connection". You could take the bus from D-MGM to, say, Caribbean Beach Resort, and then take a bus from there to Downtown Disney.
Just be aware that under extreme circumstances, the boats to DTD has been restricted, providing priority to resort guests of specific resorts over others. It happens very infrequently but it does happen.
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