Yes, there is no reason why you could not have a Gunter rig on a Winder.
One point, the mast step on a gunter rig boat is normally about 25mm further forward than on bermuda rig boat (this is to ensure the mainsail stays in the same place).
The mast step platform on a Winder is long enough to do this (just). If you are only interested in using the boat for cruising, you may not be too concerned about having the mast step as far forward as the rules allow, in which case you should be able to use the Bermuda mast step position with your gaff rig. Set up like this the boat will probably still comply with the class rules (since there is a 30mm tolerance on the position of the mast step), but there may be slight loss in performance because the mast can't be raked as much as is possible when the mast step is further forward.
There is very little, if any, demand for new gunter masts & gaffs these days. Trident-UK , and maybe others like P&B, will be able to supply new, but these items are very cheap second hand. I mention this because when you come to sell the boat, the lack Bermuda rig will affect the resale price. It just does not look to me like good value for money to spend region of £510 (2014 prices) on a new gunter rig (mast, boom, gaff) which will have a resale value close to £0 when you could buy a 2nd hand gunter rig for a song. If you do go for new gunter rig, buy a 50mm diameter alloy boom which will work fine on a gunter rig and is the normal choice of boom for a Bermuda rig.
