I have a dual-monitor setup. When I open the gTile interface it appears on both screens, and I can select cells in the layout to place the window where I want. If I choose cells in the left monitor interface the window is placed on the left monitor, and if I select cells on the right monitor it is placed on the right monitor. So far so good.

However, if I enable the "Automatically maximize windows that cover the entire grid" option, then selecting all cells on the right-hand (secondary) monitor will result in a maximized window on the left-hand (primary) monitor. If I switch off that option then the window ends up on the right-hand (secondary) as expected.