It wasn't unexpected; he was a very old gentleman at nearly three and a half, and every time I picked him up my heart broke slightly at how light and brittle he was. On Friday he was obviously in some discomfort. I took him to the vet, who diagnosed overgrown teeth and sedated him to trim them.
calgor picked him up from the vet's for me and reported that he was wobbly but OK, then he pegged out the next day. I feel terrible that I wasn't there, and that I didn't even collect him from the vet myself.
I've had many gerbs over the years but Chip and Pin were really special: cute, sweet-natured and endlessly amusing. I've dropped Pin off at the vet's along with his brother's ashes, and soon they will be mingled forever.