Better to ask and get an answer than guess and...
Using the GMS reset does a 100% clear and reset of the phone and internal SD card; however it will not restore any stock apps or files that are deleted or changed. For example, if you flash a new rom, change a system file, delete the AT&T bloatware or similar, a GSM reset will not bring them back. I think it does remove root, but not 100% sure. This is basically the same as a factory reset (or hard reset as I used to call it)
Using Odin will flash new firmware onto the phone. From what I understand, the one-click Odin does not use the exact same firmware as what came on your phone, but 99% almost the same. It will restore everything back to the way the phone was when you first got it, files, settings, bloatware and unroot. Be warned, using Odin may eliminate your ability to update the phone over the air. But there is a workaround out there to kinda fix that. Odin also requires a computer.
Before you do anything I recommend to root your phone (with nothing else done), using Rom Manager create a backup of your unmodified phone, and back that up. That way if anything goes bad, just go back to a real stock. From what I have read, anyone that did that instead of the Odin one-click was still able to get the OTA update.
Please make sure and read (I know it is a lot) the entire thread about rooting and Odin before you do anything; there were too many people that just read the first and last page of a thread and missed the warning and issues.