Idk if I'd use Sengalu pinning Garp down as factual evidence that he is weaker. Garp was always implied to be stronger than Sengoku, I'd find it odd for his power to diminish so much that he would be weaker than a much older Base Sengoku at Marineford. Garp was about WB age if I'm not mistaken. Garp also had the intention of killing Akainu, and Sengoku had to act to make sure nothing happened instead of allowing the events to play out. It's also possible that Garp stopped trying after being pinned, understanding that he couldn't launch an attack on an Admiral.