My friend Boris and I tested ggate on FreeBSD over the 100Mbit/s LAN in the ISIS Lab at Poly. We measured 6.3MB/s for dd'ing a block device over ggate like this: 
/dev/da0 <--> ggated <~~network~~> ggatec <--> dd <--> /dev/null
We also exported an encrypted disk over ggate and saw a 3.6MB/s read speed like this: 
/dev/da0 <--> ggated <~~network~~> ggatec <--> geli <--> dd <--> /dev/null
The machine running ggated was a 2.8GHz Pentium 4. The machine running ggatec was a dual 600MHz Pentium 3. The disk that was being exported was an external firewire disk that can be read locally at about 33MB/s. We played with the buffer sizes with ggate and saw only small effects unless we made the buffers much smaller that the 128KB default. Both machines were running FreeBSD 6.1 kernels.

For comparison purposes, I have observed 10MB/s with NFS (copying a big file) and 8.4MB/s with netcat. For this purpose, netcat is invoked as follows: 

machine_1$ cat /dev/zero | nc -l 14004 

...and then

machine_2$ nc machine_1 14004 > /dev/null