he was 2 meters away from it before he knocked it away. So 2 meters + a few meters is basically extremely close to the epicenter of a blast that is potentially a kilometer in diameter.
That is what I'm saying. He was extremely close, but not close enough to take on the full force of the blast.
I agree that the time for him to prep the technique takes a little while, but even a glancing blow will still do damage to him as well. The damage will cause pain to weaken significantly despite gai also weakening due to the gate's effects. Although kisame was able to survive this so called 1-hit kill because of his extreme durability, the deva isn't as durable and will eventually collapse after multiple hirudoras.???? hirudora was being compressed and traveling towards kisame at the same time, the blast has to travel to deva in order for gai to make it explode in his face. You also have to take into the account of the amount of time it takes to prep hirudora. Gai has to first pound his hand, which forms the shockwave and then send it forward. He will be extremely hard pressed to do this in the five second interval while he is still being blown back by shinra tensei.
Deva will get caught in the shockwave, but it won't hit him cleanly at all like it hit kisame since deva is too fast and will try to dodge. The shockwave will hurt him, but his durability is great enough to handle many hirudora's that don't directly hit him.
Kakashi survives only 3 shinra tenseis before his durability tops out. Gai in gates has less durability than kakashi since gates lower a person's durability. Deva uses 3 shinra tenseis to defeat gai, while gai uses 2 hirudora's within the intervals which are dodged and shockwave fails to put deva down.