Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 32: The History of Kuśanābha and His Daughters
Text 1.32.7

एषा वसुमती राम वसोस्तस्य महात्मनः।
एते शैलवराः पञ्च प्रकाशन्ते समन्ततः॥

eṣā vasumatī rāma vasos tasya mahātmanaḥ
ete
śaila-varāḥ pañca prakāśante samantataḥ

eṣā = this; vasumatī = land on the bank of Śoṇā; rāma = O Rāma; vasoḥ = belongs to Vasu; tasya = that; mahā-ātmanaḥ = great soul; ete = here are; śaila-varāḥ = great mountains; pañca = five; prakāśante = shining; samantataḥ = on all sides.

O Rāma, this land on the bank of Śoṇā belongs to that great soul Vasu. Here are five great mountains shining on all sides.

Girivajra got its name because of the five great mountains.