I have been enjoying the Strada mod on my GS/3 for over a year now.
Recently I have noticed the group pressure gauge dropping during the shot. In my machine it takes about 7 seconds to ramp up to 9 bar after I push the paddle all the way to the left. In the past the pressure would hold at 9 bar until I tapered off the pressure by slowly pushing the paddle back to the right. Now it drops pressure by itself; sometimes down to 6 bar after 15 seconds.
It is possible this is an indication my beans are getting old; the shot I just pulled today was from a roast date of 1/15. But I have not noticed this behavior in a the past.
Thoughts?

It sounds to me that your "O" rings may need to be replaced.

See here :
La Marzocco GS3 MP O-rings replacement schedule??

Have you noticed any water dripping down from the top of the group head underneath the black cover?

It could be stale beans that need to be ground finer. One easy way to figure out if it's beans or seals. Insert a backflush disc and see what happens to the pressure over a period of 30 seconds. It should ramp up and sit at 9 bar until you move the paddle to the right to stop the pull. If you see the same drop you do while pulling shots, i.e. abnormal behavior, it's probably time for seal replacement.

Merle, spot on thanks.
With the back flush disk the pressure holds a steady 9 bar with no drop.
So I'm not using my coffee quickly enough

Update:
I was away for a week and when I came back the pressure would not hold at 9 bar while brewing, and while doing a cleaning flush the flow seemed very slow.
I changed all the o-rings and now things are back to normal. Better tasting coffee than the past several months; flow is back up. The machine even sounds different while brewing; a slight whistle that sounds like freer flowing water.
So even though there was no leak, the old o-rings were dramatically reducing flow rate. I think they were about a year old, not sure. I leave the machine on 24/7. I will keep track now.