I'm my very first game, CoW 1.0 tutorial Clash of Nations as Romania, I encountered a gold user playing as Southern USA. I picked up on the gold use pretty early because his technology was rapidly outstripping mine despite the fact that I had a research going continuously. I played the game, as best I knew how at that point, and was doing rather well. I was becoming a significant force in Europe.
I stood alone against the confederates and their Finnish ally. My force was primarily armored at this point and I had very little navy to speak of. It wasn't long before an invasion fleet shows up off Yugoslavia. I elected to push into Finland in an attempt to make it a 1-on-1 fight. They came at me with everything and ended up barely falling short of taking my capitol. I began a counterattack and was pushing into former Russian territory when a stack of bombers appears from thin air followed by three railroad guns and a stack of armored forces.
I had no answer for it and it would be days before I could build a counter for it. Inexperienced, I pushed on in a vain attempt to dislodge this existential threat. My armored forces juked and jived, avoiding air force attacks for a time before turning back and engaging the railroad guns. The assault was doomed to failure, though I didn't know it at the time, and eventually my force was destroyed. It was just a matter of time at that point.
Dejected, I took to global chat to ask why I never see anyone complaining about gold use. People complain about everything, it seemed impossible. The moderator told me "Do your best, disparaging comments about gold users is prohibited". Ok, it makes sense.
I must admit I was a bit discouraged. I thought I had played a good game, particularly for my first effort, but all my effort was washed away with a lot of help from gold use. I thought, I could have won in a fair fight. However, in the time since this game I have not encountered gold use on this scale and I have learned some important lessons to counter its use.
1. Avoid attrition warfare with gold users. Primarily this impacts artillery, air and naval forces. Gold users can heal damage at will. Engaging them at range is counter productive, the units must be engaged and destroyed completely, lest they come back full strength. Avoid combat until you have the local advantage.
2. It's the economy, stupid (Carville, not being insulting). To gold units, you need resources. Of course you can gold them up too, but that gets real expensive real fast. Do the math sometime on how much gold you would need to spend to rush 10 light tanks including the resources and insta-completing them, it's probably ten times what you would get for winning the round. To capitalize on this fact, build your own industry and expand (methodically, not recklessly). Also, attempt to take important territory from gold users when the opportunity presents. Of course, this applies to all games, but in the long run of the game, economy wins.
For instance, here's my economy on day 10 1939 campaign:

Good luck overcoming that with gold.