How long have you been an LPR? What's the resident since date on GC?
That's more important than how long your marriage is.
E.g. I'm operating with limited facts. You may have been married for 10 years, but been an LPR only for 3 years. Or you may have been an LPR for 8 years. Based on that you can pick eligibility criteria for N-400. If you've been LPR for 5 years or more with no breaks in residecy, you may be eligible to apply under 5 year rule. For that, it doesn't matter if you're married or not and you do not have to prove your marriage.