Here are the descriptions of widget and keymap according to “your blender install directory\2.80\scripts\startup\bl_ui\space_toolsystem_common.py”
# An optional gizmo group to activate when the tool is set or None for no gizmo.
This is of the type
A good example of this type can be found the Gizmo Operator Target python template.
# Optional keymap for tool, either:
# - A function that populates a keymaps passed in as an argument.
# - A tuple filled with triple's of:
# ``(operator_id, operator_properties, keymap_item_args)``.
# Warning: currently 'from_dict' this is a list of one item,
# so internally we can swap the keymap function for the keymap it's self.
# This isn't very nice and may change, tool definitions shouldn't care about this.