extension cords: length vs amps
(note: this post assumes that by "gauge" you mean AWG)

I expect the biggest issue with longer leads is likely to be volt drop, especially during startup when motors can draw substantially more than their normal running current. Volt drop during startup may cause slower startup or even failure to start which in turn can lead to overheating.

I would suggest that if you need to use a longer cord you should aim to keep the resistance of the cord no greater than that of the cord the manufacturer recommends. So if you double the length you should also double the cross sectional area (which in american terms seems to translate to reducing the AWG number by 3).