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At the end of the gmapping wiki page you can see which tf transformations are required by gmapping and which ones are provided by it.

Essentially, your robot's odometry node should publish the transform between /odom and /base_link while gmapping will publish the transform from /map to /odom. You can then get your robot's pose in map by requesting the transform from /map to /base_link. this and this tutorial could be helpful if you didn't find them already.