If a person wins a massive lottery prize, in the order of 200 million € or £ or $, and this prize is versed to the person's bank account, how can they avoid that their bank balance is gossipped upon by bank employees to the extent of it known by people outside the bank, in case the person DOES want to keep their own privacy? How can they possibly avoid that, assuming that bank employees will definitely not respect privacy laws?
Most Bank employees can't see that info
In most banks there is a team that is in charge of tracking deposits that reach a threshold, in the case that you define, they need to verify that the money that you deposit in your account is legal.
Depending in the laws in your country the bank will demand for evidence in the regard of from where it came? why do you have it? and what are you going to do with it?
Bank employees don't care
This team see this kind of amounts and even higher all day, every day, and some with really really obvious crimmminal background, they also get lottery winners every month, investors/shells that bring everything to shelter from their home country taxes, etc.
Other point is that the clients info is top secret for most banks employees, and nobody can reach that info without a justification, this means that the few that are granted access to this info are heavly monitored.
"how can they avoid that their bank balance is gossiped upon by bank employees" -- obviously, he can't. He has no control over what the bank employees may do. But the prize is likely transferred electronically, so no employee would have reason to check the account (such operations are logged and "curiosity" is, understandably, strongly discouraged).
This person might open a secondary account online, request the prize to be transferred to that account, and periodically transfer funds from the prize account to his ordinary account - if the prize account is in a different bank, even if an employee working with the ordinary account got curious, he could not pry into the fund sources' account information as he would have no access. The prize account bank employees would never see the holder and never have reason to check an account they would be unaware of (one doesn't simply run
SELECT * FROM accounts ORDER BY amount DESC LIMIT 20 on a bank's servers. The worst that can usually happen is that an employee may access a customer account he knows the number of from the teller station, pretending that the customer is at the desk).