The Gradient Sampler Blender add-on let's users create color ramps just by stroking over a reference image.
Blender minimal tested version: 2.83
A better compressed version can be seen here.
The add-on creates a new context menu entry, when a color ramp node is selected in the shader editor. When activating it, the mouse turns into an eyedropper and you should stroke over an area, that shows the intended gradient from a reference image (e.g. in the image viewer of blender)
The add-on also supports sampling gradients from other applications (e.g. PureRef). Since blender doesn't capture the mouse completely in case no button is pressed, the add-on supports a two button press workflow. You press the right mouse button within blender, then draw over to the external application. Afterwards stroke over the reference gradient having both the right and left mouse button pressed. The add-on takes care to take the shorter sampled gradient.
Blender only supports up to 32 entries within a gradient. In case the referenced gradient was longer, the add-on is resampling the gradient down to this maximum number of entries. This can lead to inaccuracies.
The add-on uses the MSS screenshot library of Mickaël Schoentgen under a MIT license.