Everything I have ever read says you have to be a citizen of the United Kingdom to be in an HP movie, so if someone had dual citizenship that would work.
Other then where it is needed like characters from other countries, I do not think they should use people outside of the UK, I rather like that rule.
Tonks is a metamorphmagi and very young Auror who helps Harry travel safely in a few parts of the book and helps at the end with other things. Despite her ability, based on the description of how she looks 'naturally' Gemma is far too pretty and not tomboy enough to be her.
Which is all a moot point as long as the casting directors haven't ditched that rule, which I highly doubt.