Confused figuring out the parameter indicating whether a MOSFET is logic
VGSth is the threshold voltage where the MOSFET only just barely turns on.
The datasheet for that IRF3205 tells you that VGSth is likely to give you a ID drain current through the MOSFET of only 250uA.
If you want to use a MOSFET as a 'logic-level' switching device, you need to know that the voltage level applied to the gate by your logic will be able to turn the MOSFET on properly and not just barely.
Probably the best way to do this is to refer to the MOSFET datasheet and look for the
ID vs VGS graph, like these:
  
The one on the left is the IRF3205, and you can see that it's not particularly good with low VGS values. I wouldn't use it as a switch with anything less than 7V available to drive the gate (probably even more).
The graph on the right is for the IRL3803, and you can see that its VGS requirement is significantly lower.