https://blog.merjis.com/2007/07/16/click-fraud-google-adwords-and-gclid/

“The “(stuff)” that is added appears to be unique for each advert impression, and appears to be unique in a clever way… The first part of the ID varies rapidly and the last part varies slowly. This is clever because when you are looking for string matches, you get an early failure in the string match, helping to speed the search up – an indication that some smart people may have been working on this.”

“I’ll guess that the last part of the gclid value encodes, or more likely references in some way, the advertiser ID, the keyword, adgroup, campaign and account ID’s. The first part, that changes rapidly, is probably some combination of timestamp and instance ID or advertising channel (where the advert was published). I suspect that the account and keyword part is a database ID that delivers a row with the account ID, campaign and so on – rather than being an encoding. I suspect that the first part is a timestamp and instance ID, which will also be recorded on Google servers and will tell them when the advert impression was delivered, on which site and how long it was between that impression and the click.”

And, then there’s Matt Cutts blog with some good links:

https://www.mattcutts.com/blog/better-click-tracking-with-auto-tagging/