A few thoughts..
First, from the limited information available in your post, I am not convinced that this player is heavily using goldmarks.
- Remember that this is a 4x speed game. I don't know how long this game had gone on for, but this level of building and units is easily achievable in 1 real life week (28 in-game days) without goldmark use.
- Whether or not he has bought anything on the stock market is irrelevant and not indicative of goldmark use.
- Also, this game is very much luck dependent. For example, it could be that two of his neighbours suicide-attacked each other and he manage to swop in to capture the provinces with little unit loss (and maybe even capture some of those provinces with high-level factories intact).
- To really get greater clarity on whether he used large amount of goldmarks, we need to know how long the game has lasted, and you need to look at the newspaper to examine his development trajectory.
Second, regardless of how he managed to get the units he got, you guys have a realistic chance of defeating him. Your game is not "ruined".
- From your post, this guy does not have ANY artilleries. Given the importance of artilleries in this game, this indicates that he may not be very good at this game and that you guys have a decent chance of defeating him.
- This is a 4x speed game, he needs to sleep/work and there are several of you up against him. Even if the time window is narrow, there would still have a sufficient time window for you to strike him when he is unavailable. Use artilleries to weaken his forces with no losses. Maybe the effects of this isn't too obvious in a single day, but it adds up over a few real life days.
- If there are multiple players fighting him, you all have a natural advantage due to the morale penalty from war.
- With good tactics and strategy, you can take out his mechanical units. Tanks can be countered by artilleries, you can split stack against his planes (while you gradually build fighters to counter his airforce). Railguns are relatively weak (mostly just for psychological effect). Battleships are more difficult to counter if he knows what he is doing, but battleship can only attack near the coast and has no effects inland. Read the strategy forum to learn more about the various tactics and tricks.
Finally, maybe you will still end up losing, but
- As someone already said, goldmark is part of the game
- It is only through having these challenging fights that you become better at this game. See this as a learning opportunity to get better.