If “gnothi seauton” is “know thyself”, what would “love thyself” be in ancient Greek?

OK:

ἀγάπα σεαυτόν agápa seautón. That’s the imperative. Konstantinos Konstantinides’ ἀγαπᾶν σεαυτόν agapân seautón is the infinitive “to love yourself”. The quote from St Matthew in Evangelos Lolos uses the future indicative agapēseis: “you shall love your neighbour like yourself.”

Chad Turner went with the middle voice imperative of philéō: φιλέου “be loved [by thyself]”. The verb is fine—Greek philosophers used philéō more than agapáō, and I think agapáō became more popular in Koine. But the cultural resonance of the New Testament use of agapáō is pretty strong; and while the middle voice as a reflexive is intelligible, the unambiguous reflexive seauton is a lot clearer. (If you want to use this verb in the imperative, it’s φίλει σεαυτόν phílei seautón.)