1) Those gaskets are too thin combined with a magnet that's too weak means they won't seal.
2) The gasket comes twisted into a pretzel shape so you have to spend a lot of time warming them up to de-deform them.

I first thought that perhaps I was doing something wrong with the
heating of the gasket prior to installation. I ordered these in winter last year so
decided to try when it was warmer.

It was cold and I'd never installed a gasket before. So I waited until the next summer. Got the gaskets nice and warm in the sun. Tried to get them to be fatter and held them in place to get them to be a shape that would fit. I tried stretching them from door to body after installation, holding them in place, and using a heat gun with a temperature setting.

Nope. Still too thin to bridge that gap on closing the door. Too thin and magnets too weak to get that stretch from the door to the body of the fridge complete.

I measured, and found that even after all warmed up and stretched, the gasket sold is much thinner than the old one.

Do not make the same mistake I did and don't buy that part.