Truly linear grayscale and gradients

[Ludzik]

I try to use Affinity Photo 2 to generate gradients, which has to be *mathematically correct* - it means linear gradient from white to black has to have equal distributed grey values between 0 and 255. No gamma corrections, not at all.  I can actually generate *mathematically correct* gradients by another software, but I want to mix them.  Why I try to use Affinity Photo 2.

There are two pairs of images. On both angle of line represents gradient grey level. Black lines are **true linear gradients**, red ones are gradients generated by AF2. Horizontal lines are on 100% and 0% white, vertical on 25% and 75%.   

Is it possible to set some settings to save PNG or TIFF and be sure 75% of white defined in AF2 will be exported as 0.75 white with no corrections?