Grease Pencil: Add python operator for masking with above/below layer

Adds a new python operator to Grease Pencil for adding the layer
above or below  as a masking layers to the active layer.

Creates a new file for Grease Pencil python operators in
`scripts/startup/bl_operators/grease_pencil.py`.

Pull Request: https://projects.blender.org/blender/blender/pulls/134401

scripts/startup/bl_operators/__init__.py

@@ -21,6 +21,7 @@ _modules = [
     "constraint",
     "file",
     "geometry_nodes",
+    "grease_pencil",
     "image",
     "image_as_planes",
     "mesh",

scripts/startup/bl_operators/grease_pencil.py

@@ -0,0 +1,58 @@
+# SPDX-FileCopyrightText: 2025 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+import bpy
+from bpy.types import Operator
+from bpy.props import (
+    EnumProperty,
+)
+
+
+class GREASE_PENCIL_OT_relative_layer_mask_add(Operator):
+    """Mask active layer with layer above or below"""
+
+    bl_idname = "grease_pencil.relative_layer_mask_add"
+    bl_label = "Mask with Layer Above/Below"
+    bl_options = {'REGISTER', 'UNDO'}
+
+    mode: EnumProperty(
+        name="Mode",
+        items=(
+            ('ABOVE', "Above", ""),
+            ('BELOW', "Below", "")
+        ),
+        description="Which relative layer (above or below) to use as a mask",
+        default='ABOVE',
+    )
+
+    @classmethod
+    def poll(cls, context):
+        obj = context.active_object
+        return obj is not None and obj.is_editable and obj.data.layers.active is not None
+
+    def execute(self, context):
+        obj = context.active_object
+        active_layer = obj.data.layers.active
+
+        if self.mode == 'ABOVE':
+            masking_layer = active_layer.next_node
+        elif self.mode == 'BELOW':
+            masking_layer = active_layer.prev_node
+
+        if masking_layer is None or type(masking_layer) != bpy.types.GreasePencilLayer:
+            self.report({'ERROR'}, "No layer found")
+            return {'CANCELLED'}
+
+        if masking_layer.name in active_layer.mask_layers:
+            self.report({'ERROR'}, "Layer is already added as a mask")
+            return {'CANCELLED'}
+
+        bpy.ops.grease_pencil.layer_mask_add(name=masking_layer.name)
+        active_layer.use_masks = True
+        return {'FINISHED'}
+
+
+classes = (
+    GREASE_PENCIL_OT_relative_layer_mask_add,
+)

scripts/startup/bl_ui/properties_data_grease_pencil.py

@@ -227,6 +227,10 @@ class GREASE_PENCIL_MT_grease_pencil_add_layer_extra(Menu):
         layout.operator("grease_pencil.layer_merge", text="Merge Group").mode = 'GROUP'
         layout.operator("grease_pencil.layer_merge", text="Merge All").mode = 'ALL'
 
+        layout.separator()
+        layout.operator("grease_pencil.relative_layer_mask_add", text="Mask with Layer Above").mode = 'ABOVE'
+        layout.operator("grease_pencil.relative_layer_mask_add", text="Mask with Layer Below").mode = 'BELOW'
+
         layout.separator()
         layout.operator("grease_pencil.layer_duplicate_object", text="Copy Layer to Selected").only_active = True
         layout.operator("grease_pencil.layer_duplicate_object", text="Copy All Layers to Selected").only_active = False