What is gel phase? It is caused by the heat generated during the saponification process - ie the chemical reaction between the lye and the oils. As the reaction occurs, the heat builds up and the soap begins to change in appearance. If you touch the outside of your mould you will be really surprised to feel how warm it is. It can reach 180 degrees fahrenheit during this stage.
Does gel phase matter? Well not really in so much as it will not impact on the final soap quality - you will have exactly the same product whether your soap gels or not but if you have used colourants it will affect the final colour. Gel phase tends to intensify your colours so many soap makers like to ensure their soap gels to take full advantage of this benefit.
It is also possible to get a partial gel meaning your soap may gel on the inside but not the outside - this can give a circle in the centre of your soap that is a different colour shade to the outside - it is purely aesthetic and has no impact on the final product at all.
When I teach my workshops I do touch on the subject of gel phase - it is hard not to to be honest as inevitably the soap made during the session will have begun to gel by the time it is boxed up and ready to go and the difference is very apparent from when the batter is first poured in to the mould.
The picture below shows the soap about an hour into the saponification process. It is clear to see the centre is much darker and slightly translucent with with outside still lighter coloured and opaque.
There are some scenarios where you would not want your soap to gel, for example if you are making soap with milk or honey - the high temperatures could cause scorching. If you are using an ingredient such as honey the temperatures will be higher anyway due to the sugar content and gel phase will would not be helpful. In this case you would need to keep your working temperatures much lower from the outset, and not wrap your soap or use a heat mat to assist the process. Some soapers may even put their soap in the fridge or freezer once in the mould to ensure it does not gel.
Ultimately it is very much a personal preference, in my classes we tend to use a range of natural colourants so I always suggest my students bring a towel along with them and we wrap the boxed soap up before they leave to ensure the soap does gel and we get lovely colours.