Simple RAW for 5e 4: Smackdown v. RAW

[Erys]

You cannot lose something you do not have, and nothing is put on the table

If you have 500 gold and lose 100 or gain 100, the end result is 400 or 600.

You are correct about (generally) not being able to lose what you don't already have. (Unless you have 0 successes, then you go into debt....!)

Hence why I said 'you put your bet on the table' -> because that is the amount of money (in your possession) that you are risking when you gamble.

If you gamble 100 gold, you are looking at either:
0 successes: lose another 100, or being in debt for another hundred if you don't have that money on you.
1 success: effectively losing half your bet (half comes back, you had 100- now you have 50).
2 successes: effectively winning 50% over your original amount (you had 100, now you have 150). Or,
3 successes: doubling your bet (you had 100, now you have 200).