Sunderland were well worth their 1-0 halftime lead, but blew the three points inside the opening ten minutes of the second-half.
"I thought we were brilliant in the first half - perhaps up there with the best we've played all season." Gardner told safc.com.
"The lads looked really organised and balanced; we created chances and counter-attacked and then got our goal.
"In the second half we conceded two sloppy goals and it's up to us now to go back out on the training field and work on making sure mistakes like that don't happen again."