The PC Engine GT already has native RGB, so installing a kit that accepts RGB and CSync such as the RGBDRV or LCDDVR boards is simple.

Solder the GND and 5V to the ground and 5V supplies, and the RGB pins to the CN201 header.

The CSync is inverted at the CN201 header so you have to tap off it at the non-inverted CSync from the HU6260 pin 44.

An alternative point on the same side of the board as the other wires is the top of R202/C200.

Finally, disable the old contrast circuit to fix the RGB and CSync signals as standard stable signals, by bridging the transistor Q507 top bottom right pins together (bottom right is solid 5V rail, and top is pin 8 of the LCD connector CN201).

The left of the capacitor below is connected to the bottom right transistor pin so you can solder to that for ease.

And you should get an output like this.

To adjust the image in terms of brightness, contrast and saturation I find setting all 3 dials perfectly flat is the perfect image.

You can tune them to your preference however.

You must widen the original window a lot, almost to the edges, to fit the new screen in.

Now you have a working LCD there is no need for the high voltage tube backlight circuit and its much better to disable it.

Desolder the bridge on the BLH pad to disconnect the power going to the backlight circuit and disable it. Saving a little power, but mainly making it safer inside the console without having high voltages.

Here is the final results.