Well, “[realize] how [impressive] [something] is” isn’t always easy to translate. Google Translate output is actually not so bad.

Here are a few suggestions.

J’ai levé les yeux et j’ai vu comme le plafond était haut.

J’ai levé les yeux et me suis rendu compte de la hauteur du plafond.

En levant les yeux je me suis rendu compte d’à quel point le plafond était haut.

In a narration you could go for:

Je levai les yeux. Comme le plafond était haut !

To address the grammatical questions you raised:

1. en + participe présent

This is indeed a good way of combining two actions instead of using a conjunction. The two main translations I would give of this construction are "while ____ing" (pendant que) and "by ____ing" (au moyen de).

J’ai atteint le sommet en conservant toute mon eau jusqu’au dernier kilomètre.

Another way it’s used is to correlate two verbs of which one expresses travel and the other a type of motion. This will become clearer when you compare it to English, which combines those ideas:

She skipped across the street. Elle a traversé la rue en sautillant.

Knowing that, did GT use this construction well?

Not really. It says « J’ai levé les yeux en voyant … », i.e. "I looked up while seeing" or "I looked up by seeing" — neither of which makes sense.

But we can salvage the meaning if we reverse the verbs: « J’ai vu … en levant les yeux. » That yields "I saw … while / by / upon looking up." (This is similar to Stéphane’s third suggestion.)

2. à quelle hauteur

Well spotted. This is definitely a construction that trips up English speakers. In English, as you know, we ask about the degree of a property by combining "how" + adjective:

It’s tall. But how tall is it?

But in French, this produces a glaring error because combien and comment just can’t be combined with adjectives like that:

✗ C’est haut. Mais combien haut est-ce ?

So you need to paraphrase it. For example:

What is its height? Quelle est sa hauteur ?

To what degree is it tall? À quel point est-ce haut ?

And in your sentence we have a similar structure in "how high it was". Stéphane’s suggestions seem to capture it well.

Knowing that, did GT use this construction well?

It’s not bad; you can ask how high something is using « à quelle hauteur ». However, I would agree with the reading that Stéphane’s translations imply: namely, you don’t just see "how high the ceiling is" as in you learn its height, but you are impressed by "just how high the ceiling is"!

The syntax GT offered is also not ideal if you were to use à quelle hauteur; there should be an inversion: « à quelle hauteur était le plafond ».

Stéphane’s comment about GT output not being bad (despite his providing much better ones!) has some validity, in certain cases. GT uses a statistical model and the more formulaic your text is, the more likely GT will have seen the phrase or its components in the tagged corpora it uses and thus offer a good translation — good if and when it manages to make a pastiche of good human translations. But this pastiche rarely captures nuances and doesn’t apply very well to original creative writing, nor does it combine difficult constructions well. Here GT recognizes both places where it ought to use a non-literal translation, but the constructions identified as appropriate transformations of the individual English fragments are both mishandled.

But more to the point I made in my comment: I think that if you want to dissect and learn from a passage, the best starting point is your own attempt to grapple with it or an example from someone you know to be reliable, such as a well-known writer. A question like this presupposes that something is to be emulated when it is in fact to be avoided. While it wouldn’t be fair to cite the adage "garbage in, garbage out", a GT translation isn’t as useful as correctly aligning your ideas on what is good French, in my opinion. 🙂