100mg is a very low dose, it's not likely to make you feel really abnormal or high or anything, so maybe that will help your anxiety about taking it. I didn't really feel anything the first time I took 100mg it was so subtle. Some people are more sensitive than others, but 100mg is considered a low dose, I don't think it even comes in lower doses than that.
As for effects, gabapentin is such a strange drug and for me the effects really varied depending not only on how much I took, but what time of day I took it, what I ate and when I ate, and what I was doing. I could take 600mg one day and get very different effects from 600mg the next day.