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Gmapping does not publish the pose of the robot as a pose per se (within a topic), but as a transform from 'map' to 'odom'.

In order to get the pose of the robot in the map that is built you have to get the current odometry of the robot (read your odometry topic) and then ask for the transform of that odometry (that is in 'odom' frame) to the 'map' frame. The result will be a pose in 'map' frame (the coordiantes of the robot in the map)

Then, in order to visualize the robot path through the map being built you have to create a node that asks periodically for that transform, store it in some place, and publish all the stored points as a path in a given topic (you decide the name).

Key point here is to understand the difference between 'tf transform' and 'topic'. Gmapping publishes a topic named '/map' which contains the current map, but it also publishes the tf transform 'map' (to 'odom').