@johnny5,
This is for cross domain tracking on Google Analytics 4. Normally if you set up multiple domains in a single GA4 container to track, the analytics script on the website will add a _gl parameter to the url when it switches over to the new domain from any other domain within the cross domain tracking.
yourwebsite.com/?_gl=1%2Aj0iul8%2A_gcl_au%2AMTEwNTIyMDg4LjE3Mzc2MTE3MzQ.%2A_ga%2AMTI5Nzg3OTg2LjE3Mzc2MTE3MzQ.%2A_ga_35YMNME1KC%2AMTczNzYxMTczMy4xLjAuMTczNzYxMTczMy42MC4wLjA.
According to this article on google support, redirects (as opposed to direct linking) are known to strip the parameter, unless we add configurations to help preserve the parameter. I am not sure if you have done something like this before or would know how I might be able to help preserve it, including utilizing the regex features of redirection.
