If a man has both a birth name and a married name in a geni profile, geni adds the married name to both the SURN and _MAR fields when exporting to gedcom. The birth name is thus completely excluded from the gedcom file.
For women, the export works correctly.
Here are some examples. Let's say we have a man named John Smith who changes his name to Bells, this is what the exported gedcom will look like for him:
1 NAME John /Bells/
2 GIVN John
2 SURN Bells
2 _MAR Bells
This is a woman, Lisa Smith, who also changes her name to Bells:
1 NAME Lisa /Smith/
2 GIVN Lisa
2 SURN Smith
2 _MAR Bells