I'd call it an adverb, not a particle, but you've got me thinking about what makes a particle a particle. A particle, depending on the context, can function as a conjunction or an adverb or maybe an adjective, but is there a rule that makes γε a particle and αληθῶς not one? Maybe they have to be one syllable or a combination of one syllable words, but I think I have seen ἀλλά considered a particle.
This question is sub-phonemic (that is, it has nothing to do with meaning) but you've got me curious.
οὐ μανθάνω γράφειν, ἀλλὰ γράφω τοῦ μαθεῖν.