From what I remember, GFCI monitors the current differential between the hot and neutral wires. If the difference is 0 amps all is good and the circuit is happy (doesn't trip). Characteristics of a DC motor when it is operating properly can cause a differential between the hot and neutral wires causing the GFCI to trip. The trip point of GFCI can vary slightly and the differential in DC motors can vary significantly. Ma y manufacturers of power tools put the statement about GFCI in their literature to reduce the number of calls about tripping GFCI (as long as the user reads the information).
GFCI were designed to protect against electrical shock to people (like using a hair dryer in the bath tub). If your tools work on a GFCI keep using it. If not, you could install a separate outlet for those tools.