Not every run she does is going to be ultra distance! She'll probably be running several times a week, and some of those runs will be 4/5/6 miles (where she won't have her Salomon pack on...), so not every pair of leggings needs to be super-high performance! The key is probably for them to be well made (if she runs a lot, they'll go through the wash a lot) and comfortable.
For her loooooong runs, and races/events, she'll have preferred kit that she knows works for her*. You're not trying to find that pair of leggings for this present. You're (I guess) looking for something that she will enjoy wearing, that will fit and flatter, and will see her through lots of the short/mid length training runs along the way. So don't worry about all the posts that say ultra runners won't entertain the idea of wearing anything other than high-performance/specialist kit. Whilst something like Tikiboo probably isn't a good idea, you already know she wears Gymshark etc, so it sounds like she's perfectly open to 'normal' brands as part of her kit repertoire.
I'm just back from a run with friends - I'd say we class as fairly serious runners, having clocked up about 5,000 miles between the 4 of us this year - I was in Decathlon, 2 were in Sweaty Betty and one was in something from TKMaxx of an unknown brand!
(*whilst I don't do ultra events, I do run marathons, for which I have my kit of choice, and do a lot of long trail runs in the spring/summer/autumn, for which I also have kit which works best for those, but I also have Decathlon leggings, Ronhill leggings, Skins leggings, Ronhill shorts, Nike shorts, Asics leggings... basically a bit of everything)