When using a Game Gear tv-out board, it just outputs the native resolution (160×144) in the center of screen, but a lot/most games will have garbage colors and such in the overscan area.
With the current mask settings it doesn’t let you go high enough to cover the horizontal overscan, and the vertical mask is only sufficient for covering the whole vertical overscan if you’ve got it set to line 5x.
Game Gear “overscan” area when using TV-out mods is larger than a normal CRT overscan area. It’s visible even on a CRT. It’s kinda like playing Gameboy games on Super Gameboy, except instead of nice borders you’ve got garbage colors and such depending on the game.