BIL is like cash and very safe. It returns only about half of GSY but instead of buying BIL you can as well just leave your money on your broker account. With IB for example you get nearly the same interest rate as for BIL and you avoid the trading costs of the ETF.
GSY lost due to liquidity problems but I am quite sure these losses will disappear with volatility going down. Many of these Treasury ETFs are still trading below its asset value, which is like a free lunch for bigger investors in normal times, but at the moment many have other priorities.