Gareth got Glasgow back in the match with an interception try from halfway for his first Heineken Cup score, but he couldn't hold onto Dan Parks' cross kick in the last move of the game.
Gareth told today's Herald: "My touchdown was a thrill at the time, but the bottom line is that my mistake cost us the game. I missed a tackle which let them score - then I couldn't catch Dan's cross kick when all I had to do was flop over the line.
"I feel I have let the lads and the supporters down and I hold my hands up."
But Glasgow skipper Jon Petrie leapt to Gareth's defence, saying: "We blew it long before that chance was missed. Our display in the first half had been unacceptabe."