Without looking at any commentaries on this passage, I'll just write my own interpretation, based only on text itself (and possibly accidentally integrating a number of lectures I've heard on it in the past).
Let's break down the Gemara, bit by bit, and see if any of your questions remain:
אמר להם הקב"ה במאי עסקתם
God said to them (the gentiles), "What have you accomplished?"
This should answer your question "what is the point of Romans telling God that they made a lot of money" -- they were responding "This is what we accomplished."
אומרים לפניו רבש"ע הרבה שווקים תקנינו הרבה מרחצאות עשינו הרבה כסף וזהב
הרבינו וכולם לא עשינו אלא בשביל ישראל כדי שיתעסקו בתורה
They said to
Him: "Master of the world! We constructed many marketplaces, we made
many bathhouses, we amassed much gold and silver -- and we only did it
for the Nation of Israel, so that they could learn Torah!"
They answered God, listing all of their accomplishments, ending of with the claim that they did it for the Nation of Israel, so that they would be able to learn Torah.
I think this can answer your question "Meaning, how helpful is for Jewish people if there are rich gentiles?" -- they claimed that whatever they put up, it was only so that the Jews could use their infrastructure, and not bother themselves with those construction projects, so that they could learn more Torah.
They claimed that they accumulated large fortunes, only so that they could support the Torah learning of the Jews.
God doesn't buy this lie:
אמר להם הקב"ה שוטים שבעולם כל מה שעשיתם לצורך עצמכם עשיתם תקנתם שווקים להושיב בהן זונות מרחצאות לעדן בהן עצמכם כסף וזהב שלי הוא שנאמר לי הכסף ולי הזהב נאם ה' צבקות
God says to them: "You fools (lit. fools of the world; most foolish in the world)! Whatever you accomplished as a nation, was only for your own sake! You made marketplaces so you could have prostitutes, bathhouses for yourselves to lounge in. And that gold and silver that you amassed? That's not even yours! As the verse states (Chaggai 2:8): 'Mine is the gold, and mine is the silver, so says God of the Hosts.'"
God tells them that whatever accomplishments they made, they only did it out of their own self-interest.
And that gold and silver they were so proud of? That was never really their's anyway; all wealth (gold and silver) belongs to God, because everything belongs to God.
That, I think, is the answer to "what does it mean that all the "gold and silver" belongs to Hashem?"
I hope this helps you :^)
( all above translations are my own )