Apparently they have all changed a few weeks ago, and the 'GSI' has now been dropped from all DWP email addresses. I missed that announcement.
https://www.gov.uk/government/news/g...s-are-changing
They are still valid for now and will result in an automated email being sent back with the correct address.
EDIT - That dosen't seem to be happening in all cases, the 2 that I tried (still with .gsi) went through to the complaints team anyway. Their acknowledgement however had the new address without the .gsi.