"das Gerät" is the generic term. "die Geräte" can be subdivided into different kinds or sorts of "Geräte" like "Küchengeräte" (kitchen utensils or appliances), "Elektrogeräte" (electrical appliances), "Fernsehgeräte" (televisions), "Turngeräte" (pieces of gymnastic apparatus), and so on.
You cannot use either arbitrarily in any compound words. Very often it is fixed which one to use. For example it is "das Fersehgerät" (or short: "der Fernseher") for "the TV set", but "das Feuerzeug" for "the lighter". You may not switch those. And sometimes you cannot use any one of them, simply because there doesn't exist the respective fixed expression. So "das Rufgerät" doesn't exist. You may well be understood, but immediately proven to be no native speaker. When you use the words standing alone, then "das Gerät" denotes some (technical) device, whereas "Zeug" is "stuff".