Spoiler in here:
I asked Arcade to fight rather than to go back to Freeside. Arcade goes into full bug mode if as soon as you convince Orion to fight for NCR, you walk outside. I walk outside, and there he is in new armor. He walks back into the bunker. Then I go into the bunker and watch him repeatedly walk out of the bunker and teleport back in. The one thing that might be causing this, is that I do not immediately go back and talk to Judah (or whoever), after convincing Orion. This might cause the bug. It has happened twice. The result of this is that he never shows up at the final battle, which is darn inconvenient if you've been cultivating an EW companion and want him to use the good ones in the last fight.

When I have the patience and time, I will replay that sequence and make sure to talk to Judah before exiting the bunker and running into Arcade (again, I think this out of sequence action is what causes the bug).

If there is a workaround to get him to fight, I am on PC and can use console. I've teleported him to me at the battle but he just walks off.

This time I did the quest steps in a different order, talked to Judah first. It made no difference. Four Remnants show up, no Arcade to be found.

Hi Xoham,
Not sure if I can help get Gannon to the battle with you...
He IS one of the most unlikely characters to take to the ending though.
I mean, who do you want beside you - Arcade Gannon or Boone?
LoL, that's a no-brainer.
Even Veronica or Rose of Sharon might be better choices...

I CAN tell you, that they know Gannon will be the final companion for most who are approaching the ending.
Therefore they made it easy to 'break' from him - they made it to be his own idea!
If he asks you, should he go back to Freeside and the Followers? Well he's a doctor and scientist, they could probably use him...
So send him to Freeside.
He will then present you with his Gannon Family power (Tesla) armor, saying he hopes it will help you.
You then have complete sets of both Remnant (Enclave) and Tesla power armor...
(ED-E! come here and carry some stuff will ya?)

I finally went back to Boone at Level30 - he had been 'parked' in Novac since I was Level12 and was still wearing this awful Metal armor I had given him.
"Is it time?"
Oh man, d'you know how often I had said 'not yet'? I couldn't wait to tell him 'Yes it's time'.
I put Boone in Tesla, me in Remnant - I figured he doesn't need the +1 Strength buff, but it would help me!

It's possible Gannon was in attendance at the dam someplace behind you, and rapidly died.
(Maybe while you watched The Boomers fly over.)
Anyone who does not come through the door to The Legate's Camp with you, died.
ED-E would not come with me either - he lurked back at the dam's Visitor Centre (though when I went back to check him, he was set to follow).
So the game seems to have its own ideas about who does and does not qualify to make it through the Legate's Camp doorway with you...
Regards

He definitely did not die. With the PC, I can use a console command to teleport him to me. He shows up alive. Also, I carefully checked the dam area to make sure there were no corpses.
You can get ED-E to follow you if you tell it to wait, and then follow again. That fixes it.
According to the Wiki, Arcade is good with EW. Since I chose guns, I figure it would be best to put the unique EWs on a companion. Arcade's plasma defender does a pretty good job too, and least at medium range.

Anyway, it is still definitely a bug.