Gangsi () is a village in Mainpuri block of Mainpuri district, Uttar Pradesh, India. As of 2011, it had a population of 5,354, in 934 households.
Gangsi is located on the Etawah branch of the Lower Ganga Canal, at a point where it bends to the south. The Gangsi Drain, a left-side distributary of the main canal, branches off here. There is also a canal fall on the main canal at Gangsi, as well as a bridge crossing the canal.
As of 2011, Gangsi had a population of 5,354, in 934 households. This population was 54.7% male (2,929) and 45.3% female (2,425). The 0-6 age group numbered 739 (383 male and 356 female), or 13.9% of the total population. 1,113 residents were members of Scheduled Castes, or 20.8% of the total.
The 1981 census recorded Gangsi as having a population of 3,340 people, in 593 households.
The 1961 census recorded Gangsi (as "Gangari") as comprising 11 hamlets, with a total population of 2,201 people (1,171 male and 1,030 female), in 407 households and 346 physical houses. The area of the village was given as 2,961 acres.
As of 2011, Gangsi had 2 primary schools; it did not have any healthcare facilities. Drinking water was provided by hand pump and tube well; there were no public toilets. The village had a post office but no public library; there was at least some access to electricity for all purposes. Streets were made of both kachcha and pakka materials.