"I would like to thank Trajano for favourably responding to my public appeal to all like-minded people to join our party," GFP president Vijai Sardesai said.
Welcoming D'Mello into the GFP, Sardesai described him as a "voice of Goan people".
D'Mello was expelled from the Congress for six years on April 26, after he publicly questioned the failure of the then state Congress chief Luizinho Faleiro to stitch together an alliance before the Assembly elections in February.
Speaking to reporters, D'Mello said he had been "waiting and watching" the GFP after it joined hands with the BJP to form government in the state.
"This government has shown respect for secularism and communal harmony in the state," he added.
The GFP with its three legislators is part of Manohar Parrikar-led Goa government.