I just recall the G'MIC filter Shapeism > Still colored map filled with colored dots,
GIMP top menu Filters > G'MIC > a window opens > Artistic > Shapeism
Result
Result with Maximum and Minimum size equal to 10 and spacing at 0 to have regular dots, then with a vertical curve on the alpha channel to let the semi transparent dots disappear or reinforce their opacity depending if you go on the right or the left side
GIMP top menu Filters > G'MIC > a window opens > Artistic > Shapeism
Result
Result with Maximum and Minimum size equal to 10 and spacing at 0 to have regular dots, then with a vertical curve on the alpha channel to let the semi transparent dots disappear or reinforce their opacity depending if you go on the right or the left side
Patrice