Question about paying taxes on Gold Coins?

I realize that if held more than 1 year and sold at a profit, the gains made are taxable at a rate of 28% because the gold coin would be considered a collectible. That information is very easy to come by and it's all over the net. The question I have is not easily found. What if the gold coin was a gift? What if I bought the gold from my next door neighbor for cash and sold it 30 years later (I'm 40 yrs old right now) and I can't remember exactly when I bought it or how much I paid for it?!?