The gel phase of soap making is the heating stage of saponification. Once you pour your soap into its mold it will start to heat up. Gel phase starts with the soap turning translucent in the middle and then spreads out to the edges.
Check out this goat’s milk soap that is just starting gel phase.
More gel phase.
If your mold is not insulated or not able to hold heat then your soap might not gel. No worries, soap does not have to gel in order to turn out properly.
The biggest difference between gelled and un-gelled soap is the color of it. Colors can sometimes appear darker in gelled soap. Check out this soap. The darker color went through gel phase in a log mold and the lighter round soap (from same batch) did not go through gel phase. What a different in color!
Un-gelled soap can also take longer to harden enough to remove it from the mold.
Please note that some fragrances, especially floral type scents, honey and other additives, can heat your soap up. It can sometimes be impossible to prevent gel. Many soap makers choose to try and prevent gel when soaping with milk, as milk soap heats up and can overheat causing cracks and other issues.
Below is a lavender soap that I just poured into the mold. Below it is the same soap going through gel phase. Gel phase is really dark in some soaps, but in this one, it wasn’t that dark, but you can see the difference in translucency.]