It should be...
a custom brush with something like, In the dynamics options, color from gradient + track direction
EDIT:
I tried something different as Color from gradient works horizontally, but seems to not have options to put the gradient Vertically (full vertical length of the brush)
I selected your brush sample in a rough way, then Ctrl+C (copy) to put it in the clipboard
Once in the clipboard It appears in the brushes menu, which I just select to use as a brush
The clipboard brush is at the top of your brushes
With a better/stronger contrast you might be able to do a proper brush like your sample (may be in dynamics track the direction?)
Play with fade and other dynamics option, as well as hardness and other "normal" options, you should go very closed to a "real" brush
a custom brush with something like, In the dynamics options, color from gradient + track direction
EDIT:
I tried something different as Color from gradient works horizontally, but seems to not have options to put the gradient Vertically (full vertical length of the brush)
I selected your brush sample in a rough way, then Ctrl+C (copy) to put it in the clipboard
Once in the clipboard It appears in the brushes menu, which I just select to use as a brush
The clipboard brush is at the top of your brushes
With a better/stronger contrast you might be able to do a proper brush like your sample (may be in dynamics track the direction?)
Play with fade and other dynamics option, as well as hardness and other "normal" options, you should go very closed to a "real" brush