A clarification: GMail's proxy is in two pieces; your email client connects to GMail's CDN, but in order to load content from the origin it passes through a pool of servers in Mountain View. My guess is that the transcoding servers are all located there.
I set up some servers in the US and Asia and had some images proxied to them through GMail's proxy. The traceroute paths and latencies from the requesting IPs lead me to believe that the servers were in the US, most likely Mountain View.
I probably shouldn't have asserted Mountain View, as it's more of an educated guess.
Edit: here's a traceroute from Hong Kong to the Google server that made the proxy request. Is there a flaw in this method?
@hongkong:~# traceroute 66.249.88.203
traceroute to 66.249.88.203 (66.249.88.203), 30 hops max, 60 byte packets
1 119.9.72.2 (119.9.72.2) 1.168 ms 1.130 ms 1.122 ms
2 119.9.64.64 (119.9.64.64) 1.092 ms 1.058 ms 1.052 ms
3 vl902.edge3.hkg1.rackspace.net (120.136.47.19) 1.307 ms 1.205 ms 1.304 ms
4 RHI-0001.gw2.hkg3.asianetcom.net (203.192.178.65) 1.169 ms 1.148 ms 1.123 ms
5 google.gw2.hkg3.asianetcom.net (203.192.178.30) 1.782 ms 1.755 ms 1.741 ms
6 209.85.248.62 (209.85.248.62) 6.716 ms 209.85.248.60 (209.85.248.60) 17.183 ms 209.85.248.62 (209.85.248.62) 2.105 ms
7 66.249.94.31 (66.249.94.31) 81.019 ms 80.999 ms 80.966 ms
8 64.233.175.1 (64.233.175.1) 53.256 ms 53.077 ms 53.060 ms
9 209.85.245.206 (209.85.245.206) 72.528 ms 72.488 ms 72.14.239.55 (72.14.239.55) 80.903 ms
10 209.85.242.89 (209.85.242.89) 150.251 ms 148.859 ms 64.233.174.176 (64.233.174.176) 149.147 ms
11 72.14.239.80 (72.14.239.80) 215.363 ms 215.368 ms 72.14.239.82 (72.14.239.82) 201.955 ms
12 209.85.249.45 (209.85.249.45) 223.694 ms 72.14.237.119 (72.14.237.119) 211.817 ms 212.567 ms
13 64.233.174.117 (64.233.174.117) 212.505 ms 216.239.48.103 (216.239.48.103) 215.874 ms 213.109 ms
14 * * *
15 google-proxy-66-249-88-203.google.com (66.249.88.203) 211.877 ms 212.484 ms 212.371 ms
Second edit: d'oh, I meant where ever Google's main west coast datacenter is, maybe Oregon? The point being that it appears that all of the traffic goes through a single geographic location.
