As part of written English, both Gypsy and Gipsy are correct - I have no idea quite why de Havilland chose the Gipsy spelling, other than perhaps he liked the presence of a vowel in the word... Gipsy does look less clumsy to me.
In another thread, I recalled reading an old Pilot article written by Alan Bramson about Tigers in which he managed to use both spellings of the word!
Cheers
Treadders