There were certainly votes lost in Michigan over Gaza, but even if every single Jill Stein vote was a protest vote (they weren't), it wouldn't have been enough for Harris to carry the state.
The tougher thing to parse is the reason why so many voters seemingly stayed home this cycle. I think there is a very reasonable argument that not enough people were excited about her message, even the base.
It's a lot easier for door knockers, phone bankers, and everyday democrats to talk proudly about their candidate if they can rattle off a list of great things their candidate will do. It's even easier if those great things hit people where they're hurting the hardest or is the moral thing to do (healthcare for the uninsured, ending the wars in Iraq and Afghanistan, etc.). It's a lot tougher to get low propensity voters to show up on the harm reduction argument alone, especially if you brush past where they're hurting or concede too much ground on your moral positions.
The biggest issue for most voters appears to have been inflation and the economy, and while democrats were technically correct to say the rate of inflation has come down and American economic indicators outperformed most other countries in this post-pandemic period, that's all pretty meaningless to someone whose real wage growth didn't keep up with inflation these past few years. The "opportunity economy" and targeted small business tax cuts is a much tougher sell to someone working two+ jobs to get by.
The other issue that dominated the media was immigration. Democrats forfeited their moral position when they offered the republican wishlist border bill earlier this year. The argument that republicans weren't serious on the border because they didn't support the bill fell flat, and instead democrats were (rightly) criticized for abandoning their framing of the issue as a choice between deportation and amnesty, and their previous claims the border wall was racist.
All of that to say, democrats failed to connect with their own base on the issues that make them the party's best messengers. Add Gaza to the list of issues where Harris could have pivoted away from Biden, instead of running into the arms of the Cheneys to chase the mythical moderate republican voter.