I can't speak for Gold, as I've never reached those dizzy heights, but can speak of being downgraded from Silver.

I was Silver to the end of 2008, but didn't take any paid flights in 2009, just 1 award ticket. I was expecting to be downgraded to 'Privilege Club' staus, the entry level, but, to my pleasant surprise, was only lowered to Burgundy. I'm now back up to Silver, and intend to stay there.

About November 2009 I got an E-Mail from Privilege Club, noting that I hadn't accumulated any points or miles for that year. It offered double points and miles on any flights that I took between then and the end of the year as an incentive for me to retain my status. They stressed that this was a personal offer, rather than one available to membership generally, and I presume that its standard procedure to send such communication to members who's points tally for the year to date isn't enough for them to retain their current level.

Perhaps they also do this for Gold members who are in a similar position.