Prefer:

``` typedef NS_ENUM(NSInteger, GCSMissionItemUnits) {
        GCSNoneItemUnits,
        GCSDegItemUnits,
        GCSSecItemUnits,
        GCSMetersItemUnits,
        GCSMetersSecondItemUnits,
        GCSCentemetersSecondItemUnits
}````

Or at least use the NS_ENUM macro as above with your existing type names since it gives you explicit type information. See:
http://nshipster.com/ns_enum-ns_options/