There are many GPS solutions available, the details are important.
Question 1: Where does it get the power from?
The GPS does not require a lot of power but still, an external battery might make sense. Optimum obviously would be to have the option for both, camera battery or GPS own battery.
Question 2: What happens if the camera is turned off?
Most solutions keep running! No matter if it is a bluetooth solution or a GPS connected directly to it! The reason this is done that way is so you do not have to wait for multiple seconds until the GPS has a satellite fix, it is running non-stop. The downside is, you forget to turn it off - does it have a switch even??? - and a few days later your camera battery is dead. I don't like that at all. That is one of the reasons why I did not go for the blue2can but the www.foolography.com one, this one does switch off if you turn the camera off. And actually, keeping the GPS running does not make sense anyway. If you turn off the GPS it keeps the latest satellite data in memory and if it is turned on again shortly after, it takes just a few seconds to get a fix - not minutes.
Question 3: Bluetooth or direct GPS?
The bluetooth solution is times smaller, does not have any cables and can remain attached all the time. The power consumption is marginal.
The GPS receiver can be any standard one, if you want a new one no problem, if you want a GPS mouse that supports logging as well - sure, why not. You just have to pair it again. With the bluetooth solution obviously the GPS has its own battery and its own ON/OFF switch. What I have chosen is a GPS mouse that goes into a standby mode if it is not moved for 10 minutes. So I turn off the camera, turn it on again a few minutes later to take another picture - the GPS data I get immediately as the GPS receiver was never turned off. Back home I just put the camera and the GPS into its storage box, 10 minutes later the GPS shuts itself down automatically. Very neat.
The major advantage of the direct GPS is you can use one that has a built in compass and store this information as well in the EXIF data of the pictures.
Question 4: What if there is no GPS reception while taking a picture?
Imagine you walk into a house and take a few pictures there. No GPS signal shall be available here. You will see that GPS icon in the camera flashing. The alternative would be the GPS device keeps sending the last known position. Don't know what's better in such a case. Personally, if GPS information is important I check if the camera has data. And if it is not important, I do not want wrong or old data. On the other hand....