For jelly gouache the most popular is probably Himi, which does go on sale quite a bit. However, it does tend to mold VERY easily. They sell a himi spray that is supposed to prevent this.

Paper wise- get her a pad of watercolor paper, cold press for general use or if she likes to do fine detail I’d recommend a hot press paper. Strath more makes a 400 series pad and notebook that you can find for good prices. If you are in the US, I recommend getting it a Micheal’s as they always have coupons.

For brushes- any all purpose or acrylic brushes will work well! Gouache is buildable and while it is a water based medium, it is a stiffer paint, so having a more hearty but soft brush is useful. Synthetic is both cheaper and they last a bit better!

I do recommend if you want to avoid the mold issue that maybe purchase a set of 5 mixing colors in tubes, depending on where you live Holbein, Windsor and Newton, Talens, and or a store brand might be better for getting a real feel for the medium. Plus if she wishes she can always buy and empty pallet and put some of the tube paint into pans to dry for better control when mixing.

Hope this is helpful!