You're drawing a Texture using a SpriteBatch. A Texture doesn't support rotation. I suggest that the Sprite class might be better suited for what you are trying to do. Here's a rough outline of what you might do... see the Sprite javadoc for more detail.
private void createGemSprite() {
gemSprite = new Sprite(gemTexture);
gemSprite.setPosition(10, 10);
}
@Override
public void render() {
SpriteBatch batch = new SpriteBatch();
batch.begin();
gemSprite.draw(batch);
batch.end();
if (Gdx.input.isTouched()) {
rotateRight();
}
}
private void rotateRight() {
gemSprite.setRotation(gemSprite.getRotation() - 90);
}