• Paternal grandfather: Suparshva (of the Nāya/Jñatri clan). ( jainworld.jainworld.com )
• Maternal grandfather: Chetaka (leader of the Licchavis at Vaiśālī). ( en.wikipedia.org )

Notes:

• Many traditional Jain accounts name Chetaka’s consort as Subhadra (hence, Mahāvīra’s maternal grandmother is remembered as Subhadra), while the paternal grandmother’s name is not clearly preserved in early canonical texts. ( jainworld.jainworld.com )
• Both Śvetāmbara and Digambara traditions agree on Chetaka as the maternal grandfather; grandmother names vary in later narratives. ( britannica.com )