In the datasheet for a mosfet it has a gate sourse voltage of +-20. Is that 20 volts with respect to ground or above the sourse voltage? And is that the minumum voltage it takes to turn the mosfet all the way on?
Gate Source Voltage is measured from Gate to Source. It has nothing to do with ground or any other point.
The MAX voltage of +/-20 V is the maximum voltage you should ever use. The part could die at 20.5 volts. (it probably will survive 21V but might not) Don't go above 20!
There is a gate turn on voltage. which is the min. voltage you should use and expect the part to be on.
If you want the minimum gate-source voltage to fully turn on the transistor as a switch then look for the ON resistance measurement values in the data sheet. The gate-source voltage they use for that measurement is the minimum you need (not Vth which is the voltage to just start to turn it on). That value is typically 10V for standard MOSFETs and 5V or less for logic-level type MOSFETs.
It depends on the current you want it to conduct between D and S. The higher the current, the more voltage you need. Look at the graphs in the datasheet.
Yes, the first row and second column where it says "Gate Threshold Voltage". This is between 2 and 4 volts for a drain current of 250uA and between 2 and 4.5 volts for a drain current of 1mA.
For using it as a switch you need to look at the next two rows where it shows the Rds(on) and Vds(on). As you can see they use a Vgs of 10V for those measurements at higher drain currents. Thus you should use Vgs=10V minimum when it is a switch.
The data sheet curves are typical values and should not be used for worst-case design (which is the way most designs should be done).