To face a left-handed tennis player like Rafael Nadal, or like John McEnroe was a very difficult challenge for their opponents: Roger Federer has faced Rafael Nadal's left-handed game for fifteen years and, if he now has effective countermeasures, before 2017 he, too, suffered often, especially on clay. For all tennis practitioners, facing a left-handed person is always a challenge.
Brad Gilbert knows it well too. A left-handed tennis player can have advantages against a right-handed player. A left-hander normally serves better from the left side and, from that position, it is easier for him to play to the right-hander's backhand and find the right angle.
For a right-hander, the best situation against a left-hander is when he served on the right side. So what's the problem for the right-handed player? There are many more advantageous situations that are played from the left side.
It is much more difficult for a right-handed player to throw the left-handed player off the court when he serves from the left. And even if you do, the left-handed will be able to respond with the forehand, presumably his best shot.
If instead, you try to look for his backhand, you have to serve in the center of the court. To face a left-handed player, you need patience and, above all, you need to try to cheat his certainties and his confidence. The left-handed likes to serve from the outside left towards your backhand: you challenge it, stand with your feet (at least one and a half) beyond the line of the corridor leaving much more space in the center (towards your strongest shot: the forehand).
So he will be forced to change direction from what he prefers. One of Federer's problems with Nadal was a kind of psychological blockade, such as a virus (the forehand full of top-spin) that became chronic (on the one-handed backhand of Federer).
Roger often came on the court with a too worried attitude. When facing a left-handed you have to try to be a problem for the left-handed, as he is for you. By eliminating this psychological block you will have an advantage. Roger did so starting from the Australian Open 2017, playing more consciously and mentally free.
From that day Federer has won six of their seven matches. Furthermore, many right-handed players often face a left-hander with the same habits. Here you will have to change your mentality and play differently than usual, with many variations and try to be less predictable: for example playing more often in the right side, using variations such as lobs and volleys.
Another fundamental factor will be to remove the reference points from the left-handed. Change your position often in response: once you are three steps ahead, another three steps further back than usual. You force the opponent to adapt to you. Roger Federer understood this and now Rafael Nadal is no longer a chimera for him.