It's free energy, yes ... but only a relatively small amount of free energy, in the grand scheme of things.

Let's say you go as big as possible with a blue whale, topping out at 200 metric tons. And let's say the dedicated facility built for this has a height of 500m from which it can capture the energy of your descent. Each descent can (assuming 100% efficiency of the system) deliver about 980MJ (272kWh) of energy. A 500m drop in complete freefall would take about 10 seconds (disregarding air resistance). But since we need you to survive to do the process over again (rather than splatting into whale goo at the bottom), you'll need to decelerate gently at the end, which will slow the process down. Capturing energy from the fall will also slow you down. Let's say the drop takes 20 seconds, minimum. And, as unrealistic as it is, let's go for a highly optimized scenario of lifting human you back to the top just as quickly, so 20 seconds for the trip back up. Assuming you can transform instantaneously, that's 40 seconds per cycle -- 90 cycles per hour, at the absolute maximum. 90 cycles x 980MJ means you can generate up to 88,200MJ per hour, which is equivalent to 24.5MW.

A fairly average wind turbine like this one will generate 5MW at peak output. So you, as a blue whale power station, can generate a maximum power equivalent to ~5 large wind turbines. It's not totally free, either, since the machine to capture this energy will be huge and expensive to build -- likely bigger and more expensive to build than 5 wind turbines.

(Not to mention, we're ignoring a lot of minor inefficiencies here. Air resistance will be an energy-sapping factor on both the downward and upward parts of the cycle. Then there's friction in whatever mechanical setup you build for the machine. Then there's the energy cost of lifting human you back to the top. Then there's the inevitable inefficiency of the energy capture, which would more realistically be in the 80-90% range at most, not 100%.)

And to keep up that relatively meager energy production, you'll have to transform every 20 seconds. 180 times per hour, every hour, as long as you can keep doing it. And assuming you need to be awake to transform, the power station will have to go offline regularly while you nap. It could also be a problematic power source because of its pulsing nature -- 20 seconds on, 20 seconds off. That's messy power to be dumping into a power grid, forcing other energy sources to constantly vary in order to make up for it. You could smooth that out with giant capacitors or something ... but then that adds additional inefficiency and power loss into the system.

Yes, it's neat to be able to cheat physics and get 'free' energy ... but on a grid level, the amount of free energy you can possibly get this way is pretty trivial. Not worth giving up your whole life for.