Gap Gauge for Denso X27EPR-U Spark Plug

Just use a normal plug feeler gauge.

So, a flat blade gauge is fine for a u-channel gap?

Any method of gapping the plug will be fine.
But because you are using a plug that has not been tested and specifically approved, it will be up to YOU to determine what the "Proper" gap should be.

As a Famous Man once said, "Son, You're on Your Own!"   ?

Seriously, Let us know how it works

What do you mean?  It’s the exact spark plug listed in the maintenance manual for the 914.   See the attached picture. X27EPR-U9. 

I know Rotax specifies their plugs now, but I understood the X.. spark plugs were used for a long time and work fine. 
And at 1/10 of the cost of the Rotax plugs…

what am I missing?

You are correct. My apologies.

I was not aware that the 914 has a different plug from all the rest of the 900 series engines.
All the other engines use the now obsoleted NGK plugs or the new dual-electrode Rotax proprietary plug.
As you noted,  the X27...  plug has long been and still appears to be the specified plug for the 914 engine.

I would gap it using conventional methods.