Not sure how it is treated by the DL system is now, but I thought I saw a post from someone this year saying they were upgraded on one as if it was an award ticket.
In the T&C of the ticket is says "Upgrades: Not permitted. " but not sure if that means medallion upgrades, with miles, or any form of upgrade.
It would make logical sense for these to be treated as an award ticket so a GM+ is eligible, but again this is DL so logic does not apply... remember companion certs are not upgrade eligible even if both pax are GM+.