Hello,
If she was a land girl, it would most likely have said so on the marriage cert. What makes you think that she was? Many country women worked as farm labs of one sort or another, (over 260,000 of them in 1917) but official WLA women were a tiny minority of those at that time.
If she was born in Gibraltar it sounds as if her father might have been a soldier at one time and had returned to his former occupation.
There are, however, several Gibraltars in the UK. Sounds as if she was among the great many young women of the time who were "at home". Have you found her or her father on censuses? In 1911 she would have been 21.
As for what the Royal Berks Regiment might have been doing in Builth Wells, Powys? A question to that effect to the knowledgable on the Armed Forces board of this site might come up with an answer.
Actually, William Bushnell might simply have been on leave or hospitalised when he met your grandmother.
Yours
Vicwinann