Greed can make people do terrible things. I have seen once close families torn apart over settling the small estate left behind by a deceased family member. Literally fighting over pennies and crap. Finding a lost fortune like this would surly change your life. People act childish and petty over money. I would trust no one with a find like this, friends or especially family (I say this and I have some very close friends and family that I would trust with my life and would never let money come into the mix).
If I found this gold on my property that I owned I would keep my mouth shut and slowly sell it off in an untraceable way. I wouldn't "live large" with my new found fortune, or even deposit any of the money in a bank. At that point keeping your mouth shut will help you keep money, but you are treading down a dangerous path should the IRS ever get wind of it. Selling off slowing and paying taxes on it would be the way to go, you won't attract attention and will be legal. I would also do this if I found this gold on state or city property as well.
If the gold were on someone else property that is when things really get tricky. I have read and heard first hand many stories of property owners not caring and letting the hunter keep whatever he finds, but that mindset changes when a few old silver coins are found as now the owner wants to keep them all.
Once again on the friends issue, the owner could be a good friend but when you find a huge stash of gold the greed mentality may kick in and they lay claim to it all, leaving you with nothing and no legal leg to stand on. A find like this would really put your personal morals to the test: keep your mouth shut and keep it all or tell the owner and risk him keeping it all or blabbing his mouth and losing it all to whoever else comes out of the woodwork? Legally it is theirs unless you have a signed contract to split finds, even though they could not have found it without your metal detecting expertise
Personally I would have to see how well I knew the person and what kind of person that they were. A find of several million dollars worth I would probably get something on paper to the like of splitting the finds or a finders fee. I would do all of this without them knowing what had been found or the dollar amount yet. A find of $$$ may not be bad and they agree to a 50/50 split, but a find of $$$$$$ may result in them saying get the hell off my property while they strip-mine the place to find it themselves. But going the split route is going to create some noise and next thing you know everyone else will be holding a hand out laying claim to your find, in essence a double edged sword.
If the owner was a complete jackass I would be temped to keep my mouth shut and just never return. If they were a good person or friend, I would immediately get something on paper then agree upon a plan on how to convert the gold to cash. Because if you take the low profile approach and if they get caught you can guarantee that they will sing like a canary and bring you down with them.
Related story: Out here in the country there was a family owned junkyard that had been in business for well over half a century. When scrap steel went up several years ago the current owner began crushing cars and selling them as scrap. He was selling less than $10k at a time for cash and hiding the money. What finally got him in trouble was when his wife deposited somewhere around $9k in a bank. Apparently the bank was alerting the FBI of any large deposits as suspected bank robberies.
The FBI started investigating this fellow. When they raided his house they had sledge hammers as someone ratted him out to the authorities and claimed that he had money hidden in his walls. When all was said and done, he had over $3M in cash that he had not paid taxes on hidden in his house. After the legal fees, back taxes, and penalties they really stuck it to him. I don't believe that he was ever sentenced for tax evasion. The sad part is the junkyard owner was a good honest guy and very nice to deal with. He just made a bad decision and a series of events and friends ratting him out (jealousy?) took him down.
A find like this could be more of a curse than a blessing.