I like
Gillian and
Gemma.
Gemma is my favorite of the two, but
Gillian with a 'G' has its charm. It looks medieval and romantic, just like you said.
I wouldn't worry too much about the name being misspelled, because that can happen even with a common name. My husband's name is
Jeff, but people have misspelled his name, believe it or not. It's just one of those things everybody deals with at some point.
I don't know anyone named
Gillian OR
Gemma, but I live in the US. It might be different elsewhere.
I see both names in gold, with just a touch of rose-pink for
Gillian, and a flash of rose red for
Gemma.
Gillian makes me think of a woman living in medieval times, having a romance she really shouldn't; maybe she is a lady-in-waiting to the queen, and is carrying on with a duke, or something.
Gemma makes me think of a British girl at a boarding school, yearning to break free.