Mine works. But I hate to import the libraries myself.

import java.io.*;
import java.util.*;
import java.util.Collections;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();
        String emailRegEx = ".+@gmail\\.com$";
        Pattern pattern = Pattern.compile(emailRegEx);
        List<String> list = new ArrayList();
        for (int i = 0; i < num; i++){
            String name = scanner.next();
            String email = scanner.next();
            Matcher matcher = pattern.matcher(email);
            if (matcher.find()){
                list.add(name);
            }
        }
        Collections.sort(list);
        for (String name : list){
            System.out.println(name);
        }
    }
}

This example is too simple. endswith() is enough to solve this with Python and similar solutions that do not require regular expressions may be easily devised in other languages.

For day 28, a harder problem which essentially obligates the user to use of regular expressions would be a better fit.

In c++ using regex expressions, but you have to include <regex> header

int main(){

    int N;

    cin >> N;

    regex e(".+@gmail\\.com$");

    vector<string> db;

    for(int a0 = 0; a0 < N; a0++){

        string firstName;

        string emailID;

        cin >> firstName >> emailID;


        if(regex_match(emailID,e)){

            db.push_back(firstName);

        }

    }

    sort(db.begin(),db.end()); 

    for(int i = 0;i < db.size();i++) 

        { 

        cout << db[i] << endl; 

    }

    return 0;

}

kind of silly problem for mighty python (:

Pythonic:

import re
N = int(input().strip())
lst=[]
for a0 in range(N):
    firstName,emailID = input().strip().split(' ')
    firstName,emailID = [str(firstName),str(emailID)]
    if re.search('@gmail\.com$',emailID):
        lst.append(firstName)
print(*sorted(lst),sep='\n')