First, separate the body into 3 sections for transformation.

The way the core fighter/cockpit section connects to the lower body is pretty unique.

In the original setting, the S Gundam is piloted by 3 pilots.

Nicely enough, they included both a fixed cockpit/core block and a transformable core block that converts into a core fighter.
The one in the back is the fixed core block, and the one in the front is the core fighter + cockpit block.

It separates into the core fighter body and two cockpit blocks.

Underside

Alright, now let's transform the upper body~!

To transform, fold the head antenna back.

First, detach the arms. It's really hard to transform without removing them first.

Like any transforming kit, lift up the chest armor and...

The chest armor gimmick is absolutely brilliant. A perfect transformation with zero gaps.

Fold the yellow chin piece (;;) at the front down 90 degrees and...

Detach the backpack from its locked position on the back.

Swing the backpack and stabilizers forward.

Rotate 180 degrees like that and lock it in place.

Perfect lock! The head has completely disappeared.

Rotate the backpack around and plug it into the back of the chest armor.

Additional backpack transformation...

Center body transformation complete!
Once you handle it yourself, you'll be amazed at just how brilliant the gimmick is.

To transform the arms, you first need to open the arm-to-body locking pin forward.

Rotate the outer shoulder armor like that.

Fold the arm and finish the rotation, and...

The arm folds completely inside the shoulder armor and disappears~

Lower the shoulder cover all the way down and lock it in place.

Pull the wings out~

Then fold them back like that.

Pull out the extra locking pin that was hidden inside the arm.

The body and arm transformation sections lock into two holes like that, so it's nice and sturdy.

G-Attacker complete!
This thing is the ultimate example of a perfect transformation.

Now, on to the lower body transformation.

First, separate everything for a smooth transformation.

Raise the rear wings up 90 degrees and...

Spread the wings open.

Lower the heel down and raise the ankle guard up.

Pull the front shin piece forward and...

Bend the ankle and cover the top of the foot with it. You can tell the difference by looking at the foot color.

Bend the knee and slide the knee armor down to where the shin was.

Bend the knee in two stages and the leg transformation is done!
The gimmick is great, but honestly there's not much visible difference between transformed and untransformed.

Now attach the rear section and legs together and...

Plug the handle-side peg of the Smart Gun into the bottom of the body.

Connect the Smart Gun support to the side of the right leg.

Connect the support arm to the side of the left leg and the G-Bomber is complete~

And with that, the G-Attacker, Core Fighter, and G-Bomber are all done!

Together with their Zeta Plus Waverider siblings.