No. It was far too strong.
No technique should automatically boost someone to Juubi Jin levels, especially not a technique that doesn't come from the Juubi itself such as the Sharingan or Mokuton.
I know this is your opinion, but i highly disagree.
The gates only boost your power.
Dai using the 8th gate is no where close to as powerful, as gai was.
So 8 gates doesnt boost someone to juubi jin level automatically. It requires the person to already be absurdly strong and fast.
Gai was already kage level in attacks(though not neccesarily stamina, as gates drain stamina fast)
So I am perfectly fine with a technique that boosts a powerful person to absurd power levels. Especially if it is a suicidal technique highly focused technique, that only lasts a minute or 2. Its not like the juubi jin that is that powerlevel constantly.
Being a juubi jin doesnt automatically make someone invincible to attacks. 8th gate gais attacks were nowhere close to the scale that the juubi jin attacks on. However 8th gate gais attacks are strong enough to cause severe damage to a juubi jin. The speed is the largest powerup 8th gate gai got.
To the OP
I was very satisfied with 8th gate gai. I only wish the fight was a little bit longer, and involved a little more regular taijutsu. I have yet to see if I agree with kishis choice to let gai live. I dont know whether I would like it better if he died, makes a full recovery, or is crippled.