User blog comment:IamJakuhoRaikoben/Chapter 489: Universe One, Review./@comment-26451407-20160614030315/@comment-7418318-20160614102601

As Johnny said, we don't really know what happened except that it "was over in a flash".

That can mean many things:
 * Acnologia made a surprise attack and maimed Gildarts before he even knew there as an attack coming.
 * Gildarts underestimated the strength of Acno's attack and was maimed as a result.
 * Acnologia, rather than playing around like he did on Tenrou and with Irene, attacked him will his full power.
 * Gildarts was nowhere near a match for Acnologia.

There are probably other possibilities too, but those are the first few that come to mind.

But even if he was totally outmatched, he still had some more time to train since then. We've seen the other FT members get some serious power-ups in the three month training and later the 1-year timeskip training, so we have even less of an idea now on what level Gildarts is than we have before... and even that was sketchy to begin with.