The answer is "NO".
Now actually, with GFCI receptacles, it's a little more complicated than that (but not enough to help). GFCI receptacles are not circuit breakers and they don't even know how much current is going through them (unless it's unequal). In fact, due to a UL requirement, that 15A GFCI is rated for 20A through it, e.g. two 10A loads. But 30A is out of the question.
This is a silly unit because it requires a dedicated circuit, but it is only 120V. Normally a 3500W heater is wired 240V @ 15A, (twice the volts half the amps) so you can use the cheaper #14 wire. 30A@120V circuits are far more unusual than 15-20A@240V circuits, do this was sure to be a new dedicated circuit in any case. The rise@flow rate is rather poor, unsurprising given the low power requirements. It is weak even for a hand wash faucet; a shower is out of the question.
This is a classic problem with on-demand heaters. They take A LOT of power, and not grasping this, people far too often spec a too-small unit, resulting in user disappointment and a failed project.
The 120V unit being such an oddity, I imagine you bought it hoping against hope you could plug it into existing wiring. Well, you can't.
You need a dedicated circuit anyway, and the heating elements inside on-demand heaters are the cheap part, so you might as well get a more usefully sized heater of 30A@240V. Nobody ever got fired for buying that one for an office bathroom.
Or if you are willing to sacrifice the existing 120V circuit (commercial bathroom right?), you could convert the circuit to 240V and fit the largest heater it can support.