The reptile was rescued from the house of Abdul Zabbar Ali of No 2 Damanjhar village and his three sons were arrested, police said.
The three - Zakir Hussain, Nazibul Islam and Mafizul Islam - had brought the gecko to sell it in the market, they said.
Mafizul Islam confessed before police that he had bought the two kg reptile from a youth in Boko-Chaygaon area near here at Rs 500 about 15 days ago and kept it in his house to fatten it in order to make it 3.4 kg as geckos of that weight fetched high price in the international market, police added.