I've read that '게' is equivalent to '것이'.
Is that 100% true? are the following pairs of sentences the same?
그것이 진짜 예뻐요
그게 진짜 예뻐요그것이 너무 무거워
그게 너무 무거워
I'm also wondering if 것이 → 게 is a case of any wider pattern, or if it's a very specific contraction.
Yes, they are identical in meaning. It is a contraction, like "don't" in English. In formal writing, contractions are used less often, also like in English. You probably wouldn't see "don't" in a legal document, right? It's the same story.
Similar are: 이것이 >> 이게 and 저것이 >> 저게
Also, with the topic/contrast marker: (이/그/저)것은 >> (이/그/저)건 And with the direct object marker: (이/그/저)것을 >> (이/그/저)걸
A quick google search of, for instance, "는 게 어떻습니까" will show you 15,000 examples of this contraction used in formal cases. There is no rule against it, per se.
In fact, 거 is just another spelling of 것 that is used in certain circumstances.
Yet another contraction with 것 is in the probable future form, 것이다 and it's permutations, where the ㅅ and the 이 contract out, giving 거다, 겁니다, 거지, etc.
There are more.
One thing to point out is that 게 is used other ways, for example to make adverbs and passive forms from verb roots, so be careful to pay attention...in fact i found this a bit confusing when first learning the language.
Language is always changing. The object 것 (thing) and subject particle 이 (marking the subject of the clause) as you state, are being contracted. But the standard language (표준 언어) is 것이. While 게 (when used as the contracted form of 것이 is not standard language but a form of the standard adopted by generations that "made it their own".
I am also familiar with the ㅅ being dropped. As an example:
그건?? 아니야 그건 안돼! (That (near you)?? No way, that (near you) won't do!)
edit
Also consider that these two "match" well with the level of respect to the hearer in their conjugations:
그게 좋아
그것이 좋습니다
as for the "middle" level, it could go either way
그게 좋아요
그것이 좋아요
