What exactly is your use case? When you say you want to geotag photos do you mean you have to tag them manually or do you take a picture with your Samsung phone and you want to save the coordinates? The reason I ask is that the behaviour of Gallery is not great for what you need, but you may still be able to make it work if you are actually taking the photos and have location tags enabled in your camera app.
When you take a photo with location tags enabled the Camera app saves the photo with GPS coordinates. You can see this if you transfer the file to your Windows or Mac and view the full properties (EXIF tags) of the file. The issue with Gallery arises from the fact that it takes the GPS coordinates and maps to a more user friendly location (e.g. an address, point of interest, etc). So when you view the location field in Gallery it displays the mapped location rather than the actual GPS location even though when viewed on the map it is shown correctly (as per original GPS coordinates). This is most obvious with photos taken out in the nature as the location ends up as some street address even though you are in the bush. I think this is so that searching and organising photos by location is easier. Now, if you are happy to use the underlying GPS coordinates in your workflow outside of Gallery you can still do that as they are saved correctly.
Unfortunately, Gallery does not allow you to update the location of an existing photo to an exact GPS location. The reason for this is that the reverse mapping to what I described above takes place. Say you drop a pin in google maps, copy the GPS coordinates, go to Gallery, select a photo, view details, select edit, remove existing location (if there is one), then try to add a new one by pasting the GPS coordinates and searching for the location, you will get a mapped location that Gallery thinks is the best match. Then those coordinates will be saved. So this definitely won't work for what you need. I think this is an issue with Gallery. It should always respect the GPS coordinates and treat the more user friendly, mapped location separately. In other words, it would be great to have two location fields displayed against photos: 1) the actual GPS coordinates (as per EXIF attributes) and 2) mapped/inferred location which is more user and search friendly.
p.s. out of curiosity I just tested this in Apple Photos on iOS. Updating an existing photo with GPS coordinates works and correct coordinates are saved. Similar to Gallery, Photos displays a mapped/friendly location name in the location field however the correct GPS coordinates are preserved and the location pin is shown in the correct place on the map.
A quick update on this issue.. I did some more testing and there's definitely a problem there with Gallery not allowing locations to be accurately updated based on GPS coordinates. I filed an error report with Samsung with a detailed description and video capture of the issue. Hope it gets fixed in one of the upcoming updates.
