Rail to trail means railroad grade which means not very steep. It will take a lot of miles to accumulate much elevation gain. Typically, there are always exceptions to everything.

You get so many different answers because there isn't a perfect way to measure. Everything you checked is an estimate, and they're all estimated different ways.
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​​​​​​Ride with GPS is significant more than any of the rest and probably wrong. Did you go over many bridges? One thing that can happen when you use a digital elevation model (kind of map) is the database doesn't always know there's a bridge where you rode, so in those cases it will think you went all the way down the ravine and back up.

In general barometric altimeters are the best way to measure to in a bike. But changes in air pressure (which often proceed weather changes) are their weakness.

GPS isn't great at elevation, it's always close but almost never right, and jumps up and down a lot because of the way it works. It's especially bad under a forest canopy or with buildings or cliffs nearby.

I have a Garmin watch with a barometric altimeter, and it's mostly been excellent for elevation data. A lot of our better roads here go over mountain passes or to summit lookouts, all of these things with known elevation. When I hike I use topographic maps. Both of these offer a way to check the device and get an idea how trustworthy it tends to be, and the results tend to be excellent.

Some watches and computers use both a barometer and GPS, with GPS recalibrating the barometer regularly because of drift. I think newer Garmin watches default to this. It might be more accurate over long activities (backpacking trip, trail ultra marathon) but can introduce jitter which is more pronounced over shorter ones.

It's interesting the way there are two sets of fairly similar numbers. Did everybody start and finish the ride together?