Not just the “Gandiva”, Arjuna had acquired a lot of powerful weapons and skills through his penance and natural endowments but yes Karna is still considered greater warrior than him. I would let Lord Krishna answer this question:
Karna and Arjun were engaged in a lethal duel. Everyone around watched with awestricken eyes, as these two mighty warriors clashed. Volley of arrows raced from both sides. Both the warriors were equally matched. No one wished to leave the field of honor.
Arjuna shot a powerful arrow which hit Karna’s chariot. Karna’s chariot was back-swept by at least 4 feet. Arjuna had a foxy smile on his face, Karna was enraged, Krishna was impassive. Karna shot an arrow which hit Arjuna’s chariot and Arjuna’s chariot moved backwards by at least 2 feet. And Krishna started lauding Karna “Bravo! Karna, Bravo oh mighty warrior”. Karna grinned and Arjuna was exasperated.
This went on for sometime. Arjuna always made Karna’s chariot move backward by double the distance of what Karna made Arjuna’s chariot to move. But Krishna praised Karna every time and had no words of commendation for Arjuna. Arjuna finally got frustrated.
“This is just not done. Vasudev, you have become biased. I agree Karna is a great bowman but certainly not better than me. I am outstripping him. My arrows made his carriage reverse twice the distance as compared to what his arrows did to my chariot. Still you praise him, very unfair I must say” Arjuna complained.
Krishna smiled and said
“Dear Parth, you see Karna is a better warrior than you. Let me explain to you why? Every god wants you to win this Dharmayudhdha, their blessings are conferred on this chariot. The mightiest warrior of all times, Shri Hanuman is there on the flag of this chariot. And, I the master of three worlds is driving your chariot and still Karna makes it move backwards. Imagine, Hanuman Jee and I abandoning the chariot? One arrow of Karna can make you disappear in space along with your chariot. But don’t get demoralized, You are on the side of Dharma hence you will win, Karna with all his powers and talents can not beat you”