The Suns led by as many as 29 points and 22 at the final break before GWS kicked 8.1 in the final term, almost doubling their score, to win by seven points.
Sitting third after 11 rounds at 8-2, the Suns have lost three-straight games and have dropped to ninth ahead of hosting Melbourne on Saturday.
"Pretty frustrated and fair to say pretty deflated as well," Anderson, who had 42 touches, two goals and seven clearances, told AAP.
"We did a lot of things right and then just failed to play those key moments in the last quarter that let us down.
"That's probably the last bit of the game that we're missing.
"Being able to absorb pressure, understand the game moment and what it demands and being able to execute.
"The foundation's pretty good and then, not to say that we haven't done it in the past and won close games like that, but we definitely didn't do that (against the Giants)."
The Suns travel to face Essendon next week then play top-four sides Collingwood, Adelaide and Brisbane in a tough stretch that could define their season.
"We don't talk about the top-eight ... that will take care of itself," Anderson said.
"You've got to be measured ... we've had some close wins, some close losses.
"Our highs and lows aren't as big as they used to be. We'll just continue to learn and get better."
