As a matter of fact, it will work great in a guitar amp. It is not a plug-in replacement for a 12AX7, however. You'd have to design the gain stage around the tube, which isn't horribly difficult. It is a sharp-cutoff pentode and medium-mu triode in the same bottle, sort of similar to the 7199 found in some Ampegs. Here is a data sheet: http://frank.pocnet.net/sheets/093/6/6GH8.pdf
These are great 7199 subs, either by rewiring the socket or using the converter sockets now getting popular due to the 7199's rarity and high prices.
As Invertiguy mentioned above, 7199's are in some Ampeg amps.
And 7199's are in a lot of vintage Hi-Fi stuff.
6GH8 tubes are often employed by Hi-Fi guys, where a single gain stage (the pentode section) and phase inverter (triode section) can be employed as an entire preamp section in a single tube. This single gain stage, of course, does not favor guitar amps and if employed in a guitar amp, would likely be found with a 12AX7 in front of them.
6GH8 tubes (and the very similar 6U8 tube) are still very useful, even in 2014. However, there's a ton of 6GH8's out there (and zillions of 6U8 tubes) so even NOS/NIB examples aren't worth more than a couple bucks.