Johnston was responding to a rant on Twitter by the midfielder, who complained he had not been given a fair chance at Rugby Park.
In a string of tweets, the player also hit out at chairman Michael Johnston and claimed he had not been paid by the club he signed for in April.
Gabriel tweeted: "I just see video of Alan Johnstone telling that he has not seen enough of me. Be honest with me and fans you have see me training for months.
"I have train with you everyday for some months working very hard. I'm ready to play for the fans. This is why I come to Scotland.
"You dont pay my wages, my house and manager say he dont see me train when I train everyday. Unfair treatment by two Johnstones. Very strange."
But Johnston insists Gabriel must do better to have any chance of forcing his way into his plans.
He said: "It's good that he's disappointed not to be playing but it's a bit silly putting it on Twitter. That's just not done.
"At the end of the day, you are playing on merit and he has to do more himself to get in the team.
"He's obviously been a good player when you see that he has been involved in the national team.
"But you play in the first team on merit and because you've played for Nigeria doesn't mean you automatically get picked for the first team."
Johnston added: "There are rules in place that you're not meant to talk about football on Twitter or Facebook or whatever.
"I'm sure he understands the rules. I will speak to him and hopefully he learns his lesson.
"The main thing for him is that he's got to work harder to get himself in the team.
"He needs to do better. At this point, there are a lot of good young players in front of him."
Meanwhile, Johnston says he sees no reason why Kris Boyd cannot force his way back into the Scotland starting XI.
The striker was recalled to the national team squad by Gordon Strachan after a three-year absence and was an unused substitute against Norway on Tuesday night.
Johnston said: "I was absolutely delighted for him.
"It's a massive lift for him personally and also for the club to have someone represented with Scotland.
"People forget his age because of what he's done. He's only 30, he's still got a long way to go in his career.
"So it was no surprise to me that he was called back up.
"Now it's up to him to push on and get himself into the starting 11.
"Scoring goals is the hardest thing to do in football and he does it naturally, that's what he's all about.
"I've got no doubts he can get there again."
- Sports & Recreation
- Alan Johnstone