This is actually one of the most difficult rules to explain, and formal grammar style guides are of little help. One thing to know is that it isn't the rule that participial phrases *must* modify the subject of the previous clause -- only that they can. In fact they can theoretically modify any noun in the previous clause, and that's why their placement (except when at the beginning of the sentence!) is so flexible.
The question this raises, of course, is what does this "making..." clause in your sentence modify? The only other reasonable noun would be "jobs," and I think we're forced to take that as the meaning. You're right that, logically, it modifies the "gain," but the problem is that "gain" is not a noun, it's a verb, and participial modifiers, as adjectives, cannot modify verbs.
There is, officially, no such thing as an "adverbial participial modifier" -- that is, "making" CANNOT be an adverb modifying "gain." However, it seems that, at least on the GMAT, it almost can be. This sentence is a great example. Forgetting about grammar for a second, the "making..." phrase seems to modify the fact that there was such a gain, a fact never stated as a noun anywhere in the previous clause. Yet somehow, we imagine that it's there, and this "making..." phrase modifies it.
So, my advice for the GMAT is, *pretend* that there's such a thing as an adverbial participial modifier, even though there isn't. This construction is the reason why: a participial phrase, set off by commas at the end of the sentence, will often refer to the entire idea of the previous clause or of the main sentence, even if you can't comfortably nail down a noun to be its true grammatical modificant.
Sorry for the awkward explanation, but, I think for this rule, better to be awkward and accurate than pretend there's a clearer rule than there really is.
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