The gem system seems pretty obvious to me?
Someone can correct me if I'm wrong, but basically, you have a set amount of sockets per item types. Weapons have 2, 4 for 2H, chest has 2, helm/boots/gloves have 1, total of 9(they keep saying total of 8 though so not sure what's up with that). The color of the socket is based on the armor type, armor is red, ES is blue and so on, hybrid armor gives hybrid sockets(so can be red/blue gem in it). You can chrom as needed, just early game you're limited. These sockets are for skills only, not supports. So you can have up to 9 core skills(8 they say though, and you can have more with uniques, for example Tabula Rasa gives 3sockets instead of 2).
Then you have gems, which have their own sockets for supports on them. For example you do the quest early and get Ground Slam and it has 2 red sockets. That means you can put 2 supports with your Ground Slam. The sockets are always all linked. You can then slot that into any piece of armor with a red socket in the new window. You can chrom the gem itself for the colors for the supports, and you can jeweller to get more or less. If you find a better socketed gem, you can dump your current gem xp into the new one(probably via vendor recipe not sure). If you change the item, you have to resocket the gem in the new gear(if it's the same type, it's done automatically) but unlike before where you'd need to resocket your skill+3supports and have the right colors on the gear, now you just need a socket the right color for your skill, and the supports are automatically socketed together.
They also have meta skill gems, which are basically skills that lets you socket more skills into them, to emulate the current systems where you drop say 3 auras+enhance in a 4L, so you get Enhance as a meta skill(for example) and you can drop your auras in there, but up to 5 now so you can actually fit even more skills. Stuff like cast on crit and spell totem are also meta skills, you drop more skills and/or support into them, then have one button to cast everything.