High-side gate driver not working under load high current
The gate driver and the MOSFET are working just fine.

However, with a source impedance (R2) of 0.6&Omega;, you're simply not going to get full voltage with a 2&Omega; load (R1).

These two resistors form a voltage divider that gives you the 18.46 volts that you see at the output.

What exactly is the purpose of R2?