Fascinating inscription. What’s the erased word? It might perhaps be “PATRIAE” or “RELIGIONI” meaning either “For God, the Nation and the Arts” or “For God, Religion and the Arts”. Or something similar. I suggest that in this context, Gymnasium has its older meaning of school, rather than a place full of pain, sweat and Lycra.
Both words are about the right length and Google gives both versions and various translations.