How to load a map in the gmapping for the resumption of the gmapping in uncharted areas

[amburkoff]

On the first day, using gmapping, I create a corridor map and save it via map_saver.
On the second day, I need to load the map into the gmapping and continue creating the map in the unexplored rooms.
If I upload a map to gmapping using map_saver, it will simply be erased.
How to save a map from gmapping so that it can be loaded back into it and continue research in unknown areas?