In-universe we don’t know yet because “why” was never discussed (at least in season 1). The writers might never try to justify it in future seasons if it’s not relevant to their plots.
The Seldon AI on the ship seemed to be genuinely surprised about Gaal’s powers, not just that she had them but that they exist at all. So Gaals’s powers are likely a lucky accident rather than by design. That is, it’s very unlikely that Seldon knew about Gaal’s abilities in advance or that she was picked because of those abilities.
I disagree that Salvor’s (or likely Gaal’s) ability to resist the null field were part of The Plan. As I mentioned, Seldon’s AI on the ship appeared genuinely surprised to learn about Gaal’s powers. Also, the AI was not aghast to discover that Gaal was not at Terminus (which would have completely ruined The Plan if she had been key to opening the vault - Hari Seldon could not have planned for a second person with unusual mental powers to be born and present at Terminus). Finally the Vault Seldon was not shocked to find out that the vault had been opened anyway even though “Gaal didn’t make it”. Although I suppose that these incompatible reactions might merely be script holes.
There is another point. Depending on how closely the series will follow the books from now on, the very last thing that Seldon would have wanted on Terminus would have been an individual with “superpowers” like those of Gaal. Then again, the TV show already deviates significantly from the books, so who knows. In fact, in the books the “third” last thing Seldon would have wanted anywhere near Terminus would have been a Prime Radiant (the “second” last thing would have been a psycohistorian except one with sworn orders not to teach it to anybody else). This is because the books quickly establish that for psychohistory predictions to be accurate at all the individuals must act on their own free will without any knowledge of what those predictions are. The notion of having Gaal on Terminus, with a Prime Radiant, with a tickle for working out the rest of The Plan, and without Raych to steer her and/or an explicit directive to keep it low would have been ludicrous under the book’s plot line.
Now, the show has oh so many, many bewildering deviations from the source material that it’s hard to be sure, but I can tell you that if you have read the original stories it’s easy to make very reasonable guesses about where Gaal’s and Salvor’s powers will lead. Basically:
WARNING: Mayor spoilers for the Foundation novels, including the later novels:
In the novels, the Second Foundation is eventually revealed to be composed of a group of full-fledged psycohistorians with mental abilities: specifically emotion reading and altering (which can be finessed into quasi mind-reading and idea-inducing). This is very different from the future-predicting (maybe even altering) abilities of the show’s Gaal and Salvor, but the “mental power” connection cannot be a coincidence. There is also The Mule, a later major character with the same type of mental abilities. Finally later novels introduce Gaia, a planet full of not just similarly powered human but an entire biosphere of mentally linked beings.
In the books, Gaal and Salvor are completely unrelated to any those things, but it’s not hard to conclude that in the TV show Gaal and Salvor will be linked to one or more of them.
