In earlier times there was a Raakshas named Gad. Vishnu killed him, and Vishwakarmaa made a Gadaa out of his bones for Vishnu. The same Gadaa is called Aadi Gadaa (the first Gadaa). Vishnu killed Heti etc Raakshas from that Gadaa only. And because He bears that Gadaa, that is why He is called Gadaadhar.

Gadaasur is blessed whose bones are useful for Bhagavaan Vishnu and that He always carries that Gadaa which is made out of his bones. Till Vishnu keeps that Gadaa, He will be called Gadaadhar and Gad Raakshas name will be remembered along with Vishnu's name.

[Agni Puraan, 114]