how to parse json using groovy

That response is a Map, with a single element with key '212315952136472'. There's no 'data' key in the Map. If you want to loop through all entries, use something like this:

JSONObject userJson = JSON.parse(jsonResponse)
userJson.each { id, data -> println data.link }

If you know it's a single-element Map then you can directly access the link:

def data = userJson.values().iterator().next()
String link = data.link

And if you knew the id (e.g. if you used it to make the request) then you can access the value more concisely:

String id = '212315952136472'
...
String link = userJson[id].link

Have you tried using JsonSlurper?

Example usage:

def slurper = new JsonSlurper()
def result = slurper.parseText('{"person":{"name":"Guillaume","age":33,"pets":["dog","cat"]}}')

assert result.person.name == "Guillaume"
assert result.person.age == 33
assert result.person.pets.size() == 2
assert result.person.pets[0] == "dog"
assert result.person.pets[1] == "cat"