Coventry City have said that 12/13 Season Ticket holders can buy a ticket for Gillingham for their seat without committing to a Season Ticket.
More on this - http://www.ccfc.co.uk/news/article/gillingham-tickets-currently-sold-out-coventry-city-300814-1887039.aspx
The club have also said that "If you are a 12/13 Season Ticket holder who is struggling to renew or buy for Gillingham online, please e-mail us on info@ccfc.co.uk"