I hadn't heard of grapefruit seed extract so I looked it up
I hate saying this, because people love their herbal supplements, even when there's no science behind them (and even when there's a lot of science against them), but to me, it seems that the product doesn't live up to it's hype, but read and research for yourself.
The first and most readable source I read was from wikipedia
http://en.wikipedia.org/wiki/Grapefruit_seed_extract
On wikipedia, you do have to check the number and sources of the references cited, because some of the articles are more thoroughly researched and/or biased than others. It's the most readable, but you have to check other sources too, or check out some of the references.
A couple tablespoons of sea salt or a shot of vodka, vinegar, or citrus juice added to a sinkful of water would make a more reliable and effective antiseptic wash, but both could change the flavor of the produce, and I think that's the real issue. Any antiseptic strong enough to do significantly more than plain water would have to be in concentrations likely to be detectable in the flavor of the washed produce.
But water is sufficient, really it is. And there's no need to (and even some good reasons not to) sterilize produce before eating it.
Good old-fashioned running tap water is the best way to wash vegetables. There's no need to scrub hard or soak.