Unfortunately, by what you've said, it sounds like the knock on the head may be as possible a cause as stringhalt. I'd want to get that checked out too. Stringhalt is also a nervous/neurological disorder. If 'abnormality of gait' is the only thing, depending on what that means, it could also just be that she put her back out or such kicking at the new addition. Horses with stringhalt typically have difficulties backing up & in more extreme cases may actually kick or scrape their belly with every step of their hind legs. It doesn't tend to affect front legs until well advanced, and if left can also lead to blindness.
Given that she's in a new field, I'd be inclined to get her out of it immediately, back to a definitely safe one until you've ruled out possibility of stringhalt. It is thought to be a severe lack of magnesium, so not necessarily a specific plant that causes it, but often capeweed or other flat weed seems to be a big culprit - perhaps these plants are an indication of the lack of mag. It seems to be a cumulative thing - the more they eat of it the more they're likely to develop problems. Some also seem more susceptible than others, and I've known of cases where only one or 2 in a herd get it & all others are fine. Paddocks dressed with dolomite or such, and horses fed well balanced supps don't tend to be susceptible to the problem.
I've known horses who were rehabilitated & completely over it, but think it depends on how severe, how long they've been so magnesium deficient & allowed to graze problem pasture as to the prognosis. My 'Natural Horse Care' book by Pat Coleby says... Remove the horse from the offending paddock. No lucerne hay or grain. Feed 2 tablespoons of dolomite daily, add one dessertspoonful of epsom salts for 10 days only; also 3000mg magnesium orotate twice a day from the start. Immediately the horse starts to improve, cut the magnesium orotate to half. Once the animal is fully recovered, delete the magnesium orotate and restore the dolomite to the normal dose.