Why do we say praises of God if we can’t define him and its better to stay silent as the gemarah says and that we only say gadol gibor vnorah because moshe said it
The Talmud there does not say to stay silent, but that we are allowed and even commanded to use the praises as found in the Tanach and instituted in the siddur. He obviously doesn't need our praise, but how can we not praise His perfection?! He created us that we naturally want to praise greatness. Also, it's great for us (even if it's just a small part of His greatness) to strengthen our motivation to imitate Him and follow His advise and directives.