Electrical – N-Channel Mosfet has full Voltage on all Pins (IRFZ44N)

With 1 meg from the gate of Q1 to GND, and SENS25% at 50k, Q1's Vgs will be 4.76 V, while with SENS25% at 600k, Vgs will be 3.125V.

That means that Q1 is almost certainly capable of being fully turned on - albeit somewhat slowly - by SENS25%, but R2 will prevent that from happening because the higher Vgs goes, the higher the current through R2 goes, which will cause the voltage dropped across it to increase, raising the voltage on the source, partially defeating the rise in Vgs.

A solution might be to wire your circuit like this:

Where, when Q1 is OFF, 5V across R1 R3 R4 will make Vout equal to a "1" into your Raspberry Pi, and when Q1 is ON, the drop across the LED and Q1's Rds(on) will make Vout equal to a "0".

If not, I think you'll need some extra circuitry or a different way to go about it.

A fundamental misconception many have with MOSFETs is that the gate threshold is where the "magic" happens. The gate threshold is truly a threshold - it is the absolute threshold of conduction: where the device goes from totally of to jusssssssssssst a little bit on.

A meaningful diagram can be found a few pages deeper:

This shows various relationships between \$V_{GS}\$, \$I_D\$ and \$V_{DS}\$.

With a 12V \$V_{DS}\$ voltage, you can see that even with 4.5V on the gate, the MOSFET will conduct only a small fraction of the 110A that the device is rated for.

Yes, these curves are characterized for pulse behaviour but you get the idea - you need much more than the gate threshold voltage to really get the MOSFET "on" and working well.

If you want to drive a MOSFET from the GPIO line directly, you will need to find one which can sink sufficient current at that 3.3V drive level.

Consider the Fairchild FQP30N06L device:

You can see that even with 3V, the device will conduct a lot of current.