Gātha is a Sanskrit term for "song" or "verse", especially referring to any poetic metre which is used in legends, and is not part of the Vedas but peculiar to either Epic Sanskrit or to Prakrit. The word is originally derived from the Sanskrit/Prakrit root gai, which means, to speak, sing, recite or extol, cognate to the Avestan term gatha.
The stanzas of the Prakrit dialects of Ardhamagadhi, Sauraseni and Pāli are known as gathas as opposed to shlokas and Sutras of Sanskrit and Dohas of Apabhramsha. Most of the Jain and Buddhist texts written in Prakrit are composed of gathas (or verses/stanzas).
Thus, gatha can mean any Prakrit and Pali verses in general, or specifically the arya meter of Sanskrit, versified portions of Pali canon (Tipitaka) of Theravāda Buddhism are also specifically called gathas.
In contemporary Buddhist practice as popularized (and derived from the Zen and Theravādin traditions) by Zen Master Thich Nhat Hanh, a gatha is a verse recited (usually mentally, not aloud) in rhythm with the breath as part of mindfulness practice, either in daily life, or as part of meditation or meditative study.
Will you try to justify the meaning, of the note you sent
this evening, to my door your not deceiving me
I'd of thought that you'd have known me better, sending
round an unsigned letter, facing you would be much better
now.
Is it just that you can't face the future with me, can't
you tell me to my face
You just took the cowards way to say good-bye, how would
you feel here in my place?
If at last I think I'm glad, to rid myself of you I'm
sad, to think about the time I let you go
Then I turn and walk away, so please don't beg for me to
stay, cause I know just what I
will say to you
Is it just that you can't face the future with me, can't
you tell me to my face
You just took the cowards way to say good-bye, how would
you feel here in my place?
Tell me to my face your leaving now