Gel happens any time the soap becomes warm enough to turn into a translucent jelly-like paste. I think some people think the gel stage is only found during saponification, but soap will become a gel any time it reaches the right temperature to turn into a gel. The "right temperature" for a given soap depends on the fatty acids and water content in the soap. Gel often happens during saponification of CP soap, but it can also happen during the "cook" when making HP soap, or even when doing a rebatch by heating soap scraps in a crock pot.
Overheating is when so much heat is generated by saponification that the soap starts to grow in size due to expansion from the heat and to water vapor and other gases being created. At best, an overheated soap will develop cracks on the top. At worst, the soap can expand so much and so violently it will overflow out of the mold (volcano). Soap on its way to overheating does become hot enough to turn into a gel, but that's kind of incidental.