cli: remove gui from command blacklist

cylc/flow/network/schema.py

@@ -1444,6 +1444,10 @@ class TimePoint(String):
 class WorkflowStopMode(Enum):
     """The mode used to stop a running workflow."""
 
+    # NOTE: using a different enum because:
+    # * Graphene requires special enums.
+    # * We only want to offer a subset of stop modes.
+
     # Note: contains only the REQUEST_* values from StopMode
     Clean = StopMode.REQUEST_CLEAN.value
     Kill = StopMode.REQUEST_KILL.value

cylc/flow/scheduler.py

@@ -855,6 +855,11 @@ def command_stop(
                 pass
         else:
             # immediate shutdown
+            try:
+                mode = StopMode(mode)
+            except ValueError:
+                LOG.error(f'Invalid stop mode {mode}')
+                return
             self._set_stop(mode)
             if mode is StopMode.REQUEST_KILL:
                 self.time_next_kill = time()

cylc/flow/scripts/cylc.py

@@ -137,8 +137,6 @@ def get_version(long=False):
         ' use cylc graph <flow1> --diff <flow2>',
     'gscan':
         'cylc gscan has been removed, use the web UI',
-    'gui':
-        'cylc gui has been removed, use the web UI',
     'insert':
         'inserting tasks is now done automatically',
     'jobscript':