gclid is used by Google as a unique key to help combat click fraud. You don't want to pass that into Omniture.
As far as what to use for Omniture, it's entirely up to you. Configure your site to look for a querystring paramater such as "cid". Typically, you would use the same parameter across all external marketing. Example being http://www.mysite.com?cid=[engine or channel]_[tracking code]
Use adCenter's dynamic text {keyword} to populate the tracking code. For example, I might use http://www.mysite.com?cid=msn_{keyword} as my adCenter tracking code.
If you're already using the getQueryParam plugin in your Omniture javascript file, then you're site is already configured to grab a querystring paramter. Talk to ClientCare or the Consulting group to help you get it going.
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