I have taken the TOEFL (for the lolz, actually, and it was only two years ago) and I've been preparing for the GMAT so far.
There is one major difference between the GMAT and TOEFL - The TOEFL tests your overall grasp of the English language from a very indifferent perspective - you have listening, speaking, writing and reading. It doesn't test logic much and it's definitely nowhere close to the GMAT in terms of how hard the actual preparation is.
So in a way, yes the TOEFL would be a subset of the GMAT in some senses, but it merits preparation in it's own way, since there are a lot more writing questions and really trivial ones at that. The vocalization part is also different, because the GMAT doesn't give a damn as to how you speak, excuse my language. The reading comprehension would be similar, and I think it ends there.
In my case, my testing center was fricking cold and I was perennially freezing and my voice was so raspy, it sounded like I could have been a part of the Transformers movie. But apart from that, I think I can talk on the subject or vouch for it, because I ended up getting a 113/120 anyway.