The word 'deaconess' didn't exist yet; Paul used 'gunaikas' because no feminine term was available

The feminine term 'deaconess' didn't come into use until a couple hundred years later, at which point women called deaconesses became common throughout church history. Paul called Phoebe a 'diakonos' using the masculine term because it was the only word available. In 1 Timothy 3:11, without a word for female deacons, Paul simply says 'the women likewise.'