Literally, "to gush" is what a lot of water moving quickly does. It means the water is excessive and uncontrollable. So a river can gush, the tap in your bathroom can gush, and heavy rain comes "gushing down". So we use this image of water when we want to describe someone who talks, usually in a positive or complimentary way, but without control. A formal synonym of gushing (the adjective) is "effusive".

"To gush," is to praise somebody or something in an over the top (excessive) way. For example: "Oh my God! I cannot believe how awesome the food was in this restaurant, it was like a little slice of heaven in every mouthful, I think I want to live in this restaurant and I never want to eat anywhere else." You can probably find some videos, online of people gushing... so you could pick some using simple vocabulary that ESL students may already know, and show them a sequence of videos in which people are gushing and then ask your students to say what they feel about how the people in the videos are behaving - they might say that the people seem really 'happy, very excited etc and they might say that they are saying a lot of words very quickly.' If you can elicit this sort of description then you could put the label on it "These people are gushing." If you cannot find videos, then write your own "gushing speeches," and either read them to the class yourself (in an appropriate tone (happy/excited) and at an appropriate speed (quickly) or, if they are using simple enough words, ask students to read them out loud to the class, or work in pairs or what have you. Alternatively, you could ask each student to tell you about their favourite x, y or z... if their English is good enough they may naturally 'gush,' at you when talking about their favourite x, y or z.... you could then ask the whole class 'How did we all sound when talking about our favourite things?" etc After you have given the definition of 'gushing,' then you can concept check it with questions such as "If I am gushing am I bored?" "If I am gushing am I excited/happy etc?" "If I don't like something, will I gush about it?"