Yes! It is possible for Greninja to get Gunk Shot, but only through Move tutor in ORAS! Only Frogadier and Greninja are possible to learn this move (from the Froakie family)!
Source Here You might have to scroll down a little bit to see its move tutor moves, but I will paste them from the source here as well!
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Index: 657 - Frogadier
TM/HM: 06, 10, 12, 13, 14, 17, 18, 21, 23, 27, 28, 32, 39, 40, 42, 44, 45, 46, 48, 49, 55, 56, 62, 80, 86, 87, 88, 89, 90, 94, 97, 98, 100, HM01, HM03, HM04, HM05, HM06, HM07,
Other Tutors: Hydro Cannon,
TutorSource1: Bounce, Low Kick, Gunk Shot, Ice Punch,
TutorSource2: Icy Wind, Dark Pulse,
TutorSource3: Snore, Role Play, Water Pulse,
TutorSource4: Spite, Snatch, Endure,
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Index: 658 - Greninja
TM/HM: 06, 10, 12, 13, 14, 15, 17, 18, 21, 23, 27, 28, 32, 39, 40, 42, 44, 45, 46, 48, 49, 55, 56, 62, 68, 80, 86, 87, 88, 89, 90, 94, 97, 98, 100, HM01, HM03, HM04, HM05, HM06, HM07,
Other Tutors: Hydro Cannon, Water Pledge,
TutorSource1: Bounce, Low Kick, Gunk Shot, Ice Punch,
TutorSource2: Icy Wind, Dark Pulse,
TutorSource3: Snore, Role Play, Water Pulse,
TutorSource4: Spite, Snatch, Endure,*
Hope this helped! :)
