The Kubota wiring diagrams which show GP wiring size show 5 mm cross section (which is the metric way of sizing) as the size used.
This size does not have an exact American Wire Gauge comparison.
Your GP circuit is fused for 30 amps. A typical design would look for a voltage drop of 2% or less over the length of the wire when it is carrying 30 amps.
A 12 gauge wire 4.5 feet long would meet the 2% voltage drop.
A 10 gauge wire 7.5 feet long would meet the 2% voltage drop.
12 gauge will be adequate.
Dave