If the universe is incomplete it means that logic and science cannot tell us what lies outside of the system aka outside of the universe. We can only extrapolate general proprieties. For example we know it's not more universe meaning it is not time, space, energy, or matter.
You said yourself what GIT shows, it doesn't show that there must be something outside the universe, you just said it? We don't know if there is something outside the universe to begin with.

We can deduce as Perry Marshall does that whatever is outside of the universe is boundless, immaterial, indivisible and an uncaused cause. These basic properties match very well with the religious concept of God but this is not the only possibility. Nihilist believe that there is nothing outside the universe. Infinite nothingness could perhaps be argued to also fulfill these criteria.

Why? How are we deducing this?

“Anything you can draw a circle around cannot explain itself without referring to something outside the circle – something you have to assume but cannot prove.”

You can draw a circle around all of the concepts in your high school geometry book. But they’re all built on Euclid’s 5 postulates which are clearly true but cannot be proven. Those 5 postulates are outside the book, outside the circle.

You can draw a circle around a bicycle but the existence of that bicycle relies on a factory that is outside that circle. The bicycle cannot explain itself.
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Here’s what it means:

Faith and Reason are not enemies. In fact, the exact opposite is true! One is absolutely necessary for the other to exist. All reasoning ultimately traces back to faith in something that you cannot prove.
All closed systems depend on something outside the system.
You can always draw a bigger circle but there will still be something outside the circle.
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Now please consider what happens when we draw the biggest circle possibly can – around the whole universe. (If there are multiple universes, we’re drawing a circle around all of them too):

There has to be something outside that circle. Something which we have to assume but cannot prove

The universe as we know it is finite – finite matter, finite energy, finite space and 13.7 billion years time

The universe is mathematical. Any physical system subjected to measurement performs arithmetic. (You don’t need to know math to do addition – you can use an abacus instead and it will give you the right answer every time.)

The universe (all matter, energy, space and time) cannot explain itself

Whatever is outside the biggest circle is boundless. By definition it is not possible to draw a circle around it.

If we draw a circle around all matter, energy, space and time and apply Gödel’s theorem, then we know what is outside that circle is not matter, is not energy, is not space and is not time. It’s immaterial.

Whatever is outside the biggest circle is not a system – i.e. is not an assemblage of parts. Otherwise we could draw a circle around them. The thing outside the biggest circle is indivisible.

Whatever is outside the biggest circle is an uncaused cause,because you can always draw a circle around an effect.