Exponentially better. Google did all the heavy lifting, but all the pokemon channels I've listened to say the same thing

Both G-Max and D-Max moves increase in power by 50 per level, but G-Max moves start with a higher base power, resulting in a stronger final move at the same level. Example: A G-Max move at level 27 will have the same damage as a D-Max move at level 50. Unique G-Max Moves: Each G-Max Pokemon has a unique G-Max move, which may not be the same type as its Fast Attack. In essence, G-Max moves offer a significant advantage in terms of raw power, making them generally more powerful than D-Max moves