What is gatcha short for? Is it standard English, or is it used in the spoken language only?
Gotcha and I gotcha are relaxed pronunciations of "I['ve] got you", usually referring to an unexpected capture or discovery. Gotcha is a common colloquialism meaning to understand or comprehend.
It is the reduced written form of got you = got + -cha
Gotcha can also be spelled as gotchya whereas the related term, getcha, is made by joining the verb and pronoun, get you, with -cha.
There are no written instances of "gatcha".
My guess is that the OP has heard the Blondie song, "One Way or Another" recently covered by the British boy band, One Direction and mixed "gotcha" with "getcha". :)
One way, or another, I’m gonna find ya I’m gonna getcha getcha getcha getcha One way, or another, I’m gonna win ya I’m gonna getcha getcha getcha getcha One way or another, I’m gonna see ya I’m gonna meetcha meetcha meetcha meetcha One day, maybe next week I’m gonna meetcha, I’m gonna meetcha, I’ll meetcha I will, drive past your house And if, the lights are all out I’ll see who’s around Let's Go!