Getting map -> odom transform without using Gmapping

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In the map -> odom transform that gmapping provides, where will it get the map data? From the map on the map server or on the map that gmapping has

gmapping make its own map.

So in calculating the transform map -> odom, instead of using the map that I've loaded on the map server, (which is what I want) it will instead use the map that it has created.

gmapping will always create and use its own map.

So if I want to know the location of the robot in my map that I built with gmapping, would it matter that gmapping uses its own map to calculate the map -> odom transform?

gmapping will only give you the position in the map that it builds right now. If you want to localize in a previously recorded map, use something like amcl. See @Ben_S answer for that.