The other colors tend to cover the grey/gray curve.
UseImage = Import["https://i.stack.imgur.com/lTsCA.png"];
Use the color picker to determine that the gray curve has the following color:
UseColor = 0.5137254901960784; UseColor = RGBColor[UseColor, UseColor, UseColor];
Then look for the positions of that color in the image:
UsePositions = ImageValuePositions[UseImage, UseColor];
These are all the points we can find that are the color of interest. To ignore the legend, use:
UsePositions = Select[UsePositions, (#[] <= 400 &)];
Note the missing points. You can filter and then scale these points to match the axes. Then interpolate using a spline or linear model.