IRF3205 MOSFET gate threshold voltage higher than expected
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.