Just a guess, but...
It appears there may have been a falling out between them. There is no mention of M&M anywhere in the
History section on Gunboat's web site. In fact, it infers that Peter Johnstone designed Gunboats:
"...Peter set out to develop his own 62' catamaran. Simply put, the 62' would apply proven durable race boat technology towards the ultimate cruising boat. TRIBE launched in 2001. She exceeded performance expectations, handled like a sportscar, sailed upwind at 15 knots, and downwind above 30. In cruising mode, she achieved nearly 400 miles on a good day, and reliably 300 miles. With a year and half spent aboard cruising with his children, Peter returned home with refinements for further Gunboats." http://www.gunboat.com/history.php
Some possible reasons...
1) Things seemed to go well when Peter Johnstone pretty much had M&M all to himself with regards to designing performance cruising multihulls. M&M were designing cruising catamarans for
Leopard, but performance cruising cats only for Gunboat.
Then M&M came out with the Morrelli & Melvin 65, M&M Mala Conducta 62, and Shooting Star 50, probably direct competitors to Gunboat, or at least perceived so by Johnstone. Take a look:
MORRELLI & MELVIN catamaran - Aeroyacht
Morrelli & Melvin Design & Engineering, Inc.
Morrelli & Melvin Design & Engineering, Inc.
M&M also entered into agreement with the premier Lyman Morris boat yard in
Maine to build at least one of their boats while Gunboat was closing their
South Africa yard and moving production to
China. Read into that whatever you want. (Quality in the mind of the consumer?)
2)Morrelli & Melvin are so busy with
Leopard catamarans, their own designs, and now America's Cup boats (Morrelli & Melvin joined an international design team for Emirates Team NZ for the 34th America's Cup).

They really don't need Gunboat.
Maybe Peter Johnstone's feelings were hurt.
But he should do just fine; Nigel Irens has quite a bit of pedigree!
Marshall