Matching my Phone Camera's 'Brightness' Adjustment

[boddole]

Hello everyone, I've recently been given a Samsung Galaxy S6 that has a manually adjustable 'brightness' slider. Trouble is the UI is awful and while using it is sometimes necessary, it can take forever due to said software. I've been trying to mimic the adjustment in Gimp 2.10.8, but I haven't gotten it yet, definitely not just 'brightness' as the UI says - seems to be some mix of white balance, exposure, contrast, levels...maybe more.

I took a base picture (which usually auto sets 'brightness' around 50%) and maxed it out (though sometimes adjusting the slider does nothing). I was hoping someone had an idea as to how to approximate this effect, any ideas are appreciated.

[base]
https://imgur.com/JUSNuFV
[full 'brightness']
https://imgur.com/Uvc4FH4

   

   

[rich2005]

All sorts of ways to make adjustments, The most usual is Colours -> Curves and pull the curve around.

I can use a plugin to difference your two images and get a set of curves like this: https://i.imgur.com/a6ww7H4.jpg
A bit too complicated so can be simplified to this: https://i.imgur.com/crsKUa5.jpg Less is a flatter curve.
An alternative (closely linked to the curve) is Colours -> Levels https://i.imgur.com/9SOf0g1.jpg and pull the center slider to the left (1) While there try the 'Auto Input Levels' (2) Might do what you want with one click.