Hi everyone, i got a problem i've never encountered before : to make it simple, when i use the gradient tool it's not smooth as it should be, i got weird stripes patterns,
and when i use the air brush i got weirds concentric circles , (like thermic maps) and i don't know why. I tried a lot of things but nothing worked and i'm kinda lost. I don't know if it come from my laptop or from the software itself.
Thanks for anyone who can help!

With an 8-bit integer image, I can see very faint indications of banding on a gradient and this is to be expected for 8-bit integer images.

To get a better idea of banding, have your gradient layer over a white background and do New from Visible (to give a layer with gradient on white) and then put a Prewitt, All Sides Edge Detection Filter layer on it with radius values of about 10 (depending on the size of the image and the extent of the gradient).
That banding is greatly reduced if I use 16-bit integer images and removed altogether with 16-bit float images.

If the banding is obtrusive and obvious then it may be your graphics hardware.

With an airbrush on an 8-bit image, I can only see 'thermic maps' if I use an Edge Detection filter, i.e they are too slight for me to notice them.
It may be that you have particular airbrush settings that make them more noticeable or your graphics system may be emphasising them.
Again, 16-bit integer or 16-bit float should reduce or get rid of them. If not, it's probably the graphics system.

You can try Settings -> Configure Krita -> Display group-> Canvas Acceleration tab and then disable/enable Canvas Graphics Acceleration to see if that changes it in any way and/or change the Preferred Renderer (restart needed) to see if that helps.

Thanks a lot! so after trying all that it seems like this problem only occurs when i use deep color like pure black (or around that), and i really don't know why, because when using others lighter color everyting it's good

With black, it is more noticeable.
For the airbrush, I can see them with black paint and it seems to be because of the brush spacing.
This can be made smaller in the Brush Editor on the Brush Top settings, Spacing.
If you do that, you may need to reduce the paint opacity slightly to compensate for more closely spaced dabs.