It's possible that this is a respelling of Gelasio.
Hispanic First Names by
Richard Woods gives Gellaseo as a form of Gelasio that has been used in Texas.
Gelasio is the Spanish form of Gelasius, the name of a 5th-century Pope. If I am interpreting the Spanish and Italian name dictionaries I own correctly, the name Gelasius is thought to be from a Greek word meaning "jovial" or "laughing".