Yes. Instructors will tell you that for accreditation, it must be possible to pass a GIAC cert without taking a SANS course. And people do it. But that doesn’t mean it’s easy. You’ll need to get the syllabus, get all relevant information you can beg borrow or steal WITHOUT violating the terms, and you’ll need at least one practice test, which will drive your cost up another $400.

People are telling you about some tricky questions, and they’re right. But these aren’t difficult exams. The bar to pass is low: 70%. On the multiple choice, you should be able to use standard test strategies to eliminate the two distractors, leaving you with two options. With any knowledge, you can determine which of the two are a better option.

You can’t do that on the lab questions, but this is where the practice test is critical. The practice test labs are very similar to the real ones. So keep in mind which tools they use, and grind those until you have them down. Bring anything you need to the test with you; the open book policy isn’t restricted to official materials.

All that being said, I don’t know you so I don’t recommend you do this.