Diagnosis
Debris is caught between the disc brake rotor and the fixed brake caliper. Gotrax G4 uses a mechanical disc brake.
How to Fix It
Lift the front wheel. Spin it and locate the rubbing point. Loosen the two caliper mounting bolts just enough to allow movement. Squeeze the brake lever to center the caliper. While holding the lever, re-tighten the bolts. Spin the wheel to check clearance.
If That Didn't Work
The rotor itself may be bent. Use a rotor truing tool or adjustable wrench to gently bend it back straight. Work in small increments.
Heads Up
Never use lubricants on the brake rotor or pads. It will destroy braking power and is a major safety hazard.