Hullo everybody,
Willem, to me it appears to be a date: 15th. day of the month (a.k.a Purnama/Full-moon, as the moon just starts to reduce) and (Pasaran/Market-day) Wage on the Javanese Calendar.(One can probably calculate the year by working out when there was a 15th Wage, ..... IDK).
As for the 'GS', I don't know what that is, probably exactly what it looks like: the initials 'GS'.
Hope it helps (Javanese is really not my forte).
Best,