This is how I made gil for a big house.

[Sakuraluna]

this method is good to make money in a long run with some fund (if your luck is not reallyyyy badddd)
but it's mostly luck-based and not recommended for people with low fund unless you wanna go "win big or lose everything" route.
also it's better to put in 4-5 different types of junk/cheap materia because that increases the odds of getting red/crafter/gatherer materia.
please note that the outcome will always be different from the materia you used...
so in the vid you are very lucky to not get many other stat materias or elem res ones.

ps. check server's market first.. for me...i tried to make sure that my 5 try on transmutation will cost less than 1 crafter/gatherer's

I've actually done just this and gotten elemental or stat materia several times, so although it might seem to increase your odds, it isn't an absolute method of prevention for getting those types.