The GM is not the older model it is still being produced but not easy to get hold of here in the UK on ebay most of the sellers for the GM are from Japan they are not available from Amazon UK you might get some from AT themselves here in the UK.As ruffuz says, ifi audio has listed specs of the same set, just different colour variants.
What I would like to know is what is how much different to the b's does the older model sound (using a standard stereo plug not balanced) and if the ear enclosure is any larger on the older model (for my big ears)
Regarding the ear enclosures they are both the same size I have just had them side by side. The GM's can take calls from your smartphone using the supplied cable. Its horses for courses basically. I like the balanced cable on the MSR7b model because I can make use of that where it wouldn't help you at all it gives them a slightly superior sound to the GM's due to the balanced circuitry on the iFi Zen Dac. Thats the reason that I am keeping both sets because in their own right they are both very good headphones.