I think you really need to do some research on the difference between antioxidants and preservatives, and then read this thread again and I think it will make sense.
There is no such thing is a
natural broad spectrum and effective preservative. You don't need a preservative in your soap, the pH is too high for pretty much anything to live there. You said that your book said adding GSE extends shelf life. That is true. Antioxidants extend shelf life by preventing rancidity. If you are using soaps that are prone to DOS, adding GSE directly to your soaps could have some benefit. If you are using 100% OO, you probably don't need this.
Aside from all of that- I see you are in Greece. That means you need to follow EU regulations regarding cosmetics (soap is considered a cosmetic). This is a link to Cosmetics Directive 76/768/EEC:
http://www.leffingwell.com/cosmetics/vol_1en.pdf. I'm currently trying to begin selling in Italy, and I can tell you it is a pain in the butt. You will need a certified cosmetics laboratory, a technical director if you don't have a degree in something like chemistry or pharmacy, you have to have all of your recipes reviewed by the cosmetics board of your country, and all of your recipes sent off to be analyzed. Only certain things can be used as preservatives and colorants. Anything you use as a natural colorant must have some other purpose because you can't officially use it as a color unless it is on the 'list' which only includes artificial colors. Essential oils can only be used in certain percentages, and differs for each one. Many MANY things are forbidden to use as ingredients. Packaging and labeling is also strictly controlled. You'll need to figure out how to calculate your shelf life. I imagine tax laws are about as confusing in Greece as they are in Italy, so you'll probably also need to hire an accountant.
Basically, if you want to start selling, you better want to do it as a full time career, because otherwise your business expenses will sink you in a minute. It's not easy to sell soaps in Europe, unfortunately.
Maybe you already knew all of that. I didn't when I first started thinking about selling, about 6 months into soap making. Now A LOT time has passed since then, and it'll be another 3 months or so before I can start the paperwork. I've found a lab, technical director, accountant, suppliers, shops that want to carry my products, and have tweaked my recipes. It's been frustrating but it has given me time to become a much better soap maker.
The people on this forum who give advice might tell you something you don't want to hear sometimes, but they generally really know what they are talking about. And I have a bunch of books that are 'well respected' yet give inaccurate information. Just because it is written down in one place doesn't make it true. If you find it in 10 books, you can start to believe it
.
Hope this is helpful. I can sympathize with someone in the EU who wants to get into selling soap, and I wish someone would have told me all of this at the beginning. Still would have done it, but I would have perhaps been more efficient!
Edited to add: I agree strongly with what is written above. Soap is not 'natural'. It is created by a chemical reaction (saponification) between oils and lye (a chemical! made in a lab!). It is perhaps a step closer to natural than many commercial products, but it is the result of a chemical reaction. If you aren't careful with how you do it, people can get burned or have severe reactions. It's not something to take lightly, and if you don't understand this yet, you really need to soap longer and do some research. After reading many horror stories about because getting chemical burns due to poorly made soap, I always proceed with caution and am meticulous about my process, because I don't ever want to be the cause of someone getting hurt.