It's not so much creating an illusion that pulls the rug out from you after the fact as telling a lie that makes you doubt the evidence of your senses in real time.
lies by omission can not be gaslighting.
I think it's true that the term 'gaslighting' does tend to get distorted here from time to time, though I would argue that 'lying by omission' can absolutely be a part of that pattern. And BraveSirRobin's edit is apt; lying still has a great potential for traumatizing someone regardless of how we define it.
When I synthesize the various definitions of 'gaslighting' I can find on the Internet, I can't help but come to this conclusion: at the end of the day, it's pattern over time of a variety of types of 'lying' with the purpose of making someone (in our case, the BS) believe some sort of alternate reality. That can look like *so* many things, but it's a pattern, and it's with a purpose.
Some common, highly-generalized examples, I think:
1) WS gets you to believe they are in one place when they are in another (ex: "I'm going to be in a late meeting in the city today; can you pick up the kids and take care of dinner/bedtime?")
2) WS gets you to believe they are doing things that they are *not* doing (or not doing things they *are* doing) (ex: "Sorry, I can't meet you for lunch; I have this huge report to finish by 3:00!")
3) WS gets you to believe that they are doing things for reasons *other than* what are true (ex: "Can you believe my boss asked me to move my office to AP's wing of the building?? She says it's part of a whole company restructuring thing. Ugh, guess I'll start packing up my things!"
4) WS compels *you* to do things that are based on reasons/information that is not true (ex: "Hey, why don't you head to hunting camp this weekend? It would be great for you to have some time for yourself, and you may not have another chance this year.")
5) WS gets you to believe that they are having conversations with people or spending time with people they are *not* (changing contact info in phone, etc.)
There are probably so many other examples, but in my opinion, living your life with just these 5 things happening to you over time will drastically alter your perception of reality in just a few weeks/months. The longer it goes, the worse it is. If/when it all comes crashing down, and BS suddenly realizes that perhaps hundreds or thousands of little things they thought were true, and were impetuses for *their own* life choices, are actually NOT true...then BS will be traumatized.
It's so common, especially with the saturated, over-use of the term in pop culture, to want to respond to claims of 'gaslighting' with "that's just lying!" But I really believe it's more about the pattern, the intent, and how the lies cause BS to alter their life-course that makes it 'gaslighting,' (and, in turn, abusive).
My $0.02.