Jones spilled Jonathan Douglas’s tame header into his net for the game’s only goal after 57 minutes, though the Bees were the better side throughout and could have run out more comfortable winners.
There was also an unsavoury moment towards the end of the match when Reds full-back Paul Connolly was involved with an altercation with a group of home fans as he went to take a throw in.
His boss Gregory, situated on the opposite side of the field in the dugout, said he couldn’t see the incident so refused to comment on it.
“That was probably going to be the hardest game out of the remaining games we have, that was probably going to be the most difficult one tonight,” Reds boss Gregory said afterwards.
“We’ve battled and fought for everything all over the pitch, I couldn’t have asked for anymore effort from anybody.
“It had 0-0 written all over it, everything was going according to plan and a point would have been a very good result until we made a howler which has cost us dearly.
“Obviously on current form to come to somewhere like Brentford and go away with a clean sheet would have been rewarding and something we would have enjoyed, but it wasn’t to be.”
The usually reliable Reds No.1 Jones made a similar mistake a week ago to let in the winner away at Port Vale, though Gregory promised to back him, using Manchester City and England’s Joe Hart to make the point that no goalkeeper is immune from making errors.
It was a sixth straight League One defeat for Crawley Town and leaves them in a real fight for survival, just three points above the relegation places.
“We need to pick ourselves up, we’re at Milton Keynes Dons on Saturday which should be another tough fixture. They still have faint hopes of a play-off place so we’ll have to make sure we’re ready,” Gregory added.