The compressive strain effect on the magnetic ground state and electronic structure of strained GdTiO 3 has been studied using the first-principles method. Unlike the cases of congeneric YTiO 3 and LaTiO 3, both of which become the A-type antiferromagnetism on the (0 0 1) LaAlO 3 substrate despite their contrastive magnetism, the ground state of strained GdTiO 3 on the LaAlO 3 substrate changes from the original ferromagnetism to a G-type antiferromagnetim, instead of the A-type one although Gd 3+ is between Y 3+ and La 3+. It is only when the in-plane compressive strain is large enough, eg on the (0 0 1) YAlO 3 substrate, that the ground state finally becomes the A-type. The band structure calculation shows that the compressive strained GdTiO 3 remains insulating, although the band gap changes a little in the strained GdTiO 3.