DNA segment map
The DNA segment map view, accessible under the "DNA" tab in the person view, shows DNA matches with other people in the tree as segments highlighted on the person's chromosomes.
This feature does not rely on raw DNA data being uploaded to Gramps Web, but on the segment match data directly, which can be obtained from websites like Gedmatch or MyHeritage.
The DNA segment map is based on the Gramplet Addon that provides the same functionality in Gramps Desktop. Its wiki page contains more details about how to obtain the data, how to interpret it, and how to enter the data in Gramps.
The data is added as notes attached to associations (or attached to association citations), which is possible directly in Gramps Web.