#define GNT_WIDGET_FLAGS(obj)               (GNT_WIDGET(obj)->priv.flags)
#define GNT_WIDGET_SET_FLAGS(obj, flags)        (GNT_WIDGET_FLAGS(obj) |= flags)
#define GNT_WIDGET_UNSET_FLAGS(obj, flags)  (GNT_WIDGET_FLAGS(obj) &= ~(flags))

GNT_WIDGET(userList)->priv.flags = GNT_WIDGET_NO_BORDER; //WORKS
GNT_WIDGET_FLAGS(userList) |= GNT_WIDGET_NO_BORDER; //error: invalid conversion from 'int' to 'GntWidgetFlags'
GNT_WIDGET_SET_FLAGS(userList, GNT_WIDGET_NO_BORDER); //error: invalid conversion from 'int' to 'GntWidgetFlags'

typedef enum _GnWidgetFlags
{
    ...
    GNT_WIDGET_NO_BORDER      = 1 << 3,
    ...
} GntWidgetFlags;