Can someone help me by telling me what is the major gain(s) by using a 3rd party document retention/records management solution over what SharePoint 2010 offers?
I guess one of the biggest differences is that SharePoint includes so called in-place Records Management, meaning that you can declare something a record without having to move it away from the location (e.g., workspace) where it originally was. You can also have hybrid scenario and use both archiving type of RM and in place RM. More here.
I suggest spending 30 minutes and reading through the RM planning section from MSDN, I believe you should get a pretty good idea of RM functionality in SharePoint; then you can compare the functionalities with the 3rd party tool(s) you had in mind.