Try to generate a GP3 stroke using modifier.

was there for the UV but it's not implemented yet.

@@ -0,0 +134,4 @@
+    }
+
+    IndexMaskMemory memory;
+    const IndexMask strokes = ed::greasepencil::retrieve_editable_and_selected_strokes(

@@ -1129,12 +1129,12 @@ static int add_material_from_template(Main &bmain, Object &ob, const ColorTempla
   return index;
 }
 
-static bke::CurvesGeometry create_drawing_data(const Span<float3> positions,

@@ -0,0 +109,4 @@
+  const bool smooth_radius = (mmd.flag & MOD_GREASE_PENCIL_SMOOTH_MOD_THICKNESS);
+  const bool smooth_opacity = (mmd.flag & MOD_GREASE_PENCIL_SMOOTH_MOD_STRENGTH);
+
+  if (iterations <= 0 || influence <= 0) {

@@ -0,0 +181,4 @@
+  }
+
+  if (changed) {
+    drawing.tag_positions_changed();

@@ -0,0 +194,4 @@
+  if (!geometry_set->has_grease_pencil()) {
+    return;
+  }
+  GreasePencil *grease_pencil = geometry_set->get_grease_pencil_for_write();

@@ -7666,7 +7673,6 @@ static void rna_def_modifier_grease_pencil_opacity(BlenderRNA *brna)
 {
   StructRNA *srna;
   PropertyRNA *prop;
-

@@ -7683,3 +7689,3 @@
   rna_def_modifier_grease_pencil_layer_filter(srna);
   rna_def_modifier_grease_pencil_material_filter(
-      srna, "rna_GreasePencilOpacityModifier_material_filter_set");
+      srna, "rna_GreasePencilSmoothModifier_material_filter_set");

@@ -7686,2 +7691,3 @@
+      srna, "rna_GreasePencilSmoothModifier_material_filter_set");
   rna_def_modifier_grease_pencil_vertex_group(
-      srna, "rna_GreasePencilOpacityModifier_vertex_group_name_set");
+      srna, "rna_GreasePencilSmoothModifier_vertex_group_name_set");

@@ -17,6 +17,8 @@ set(INC
   ../render
   ../windowmanager
   ../../../intern/eigen
+  ../gpencil_modifiers_legacy

@@ -0,0 +163,4 @@
+                                     false,
+                                     opacities.span);
+    opacities.finish();
+    drawing.tag_positions_changed();

@@ -0,0 +177,4 @@
+                                     false,
+                                     radii.span);
+    radii.finish();
+    drawing.tag_positions_changed();

@@ -0,0 +256,4 @@
+    /*struct_size*/ sizeof(GreasePencilSmoothModifierData),
+    /*srna*/ &RNA_GreasePencilSmoothModifier,
+    /*type*/ ModifierTypeType::OnlyDeform,
+    /*flags*/ eModifierTypeFlag_AcceptsGreasePencil,

@@ -0,0 +117,4 @@
+    return;
+  }
+
+  bool changed = false;