Accylad,
I don't use DVgate, but I do have a HDV camcorder (HC1).
As you know with the HC3 you can capture DV or HDV. With the software I use (Sony Vegas) there is a different capture program for each, as it works differently. A quick look says DVgate plus does support DV and HDV, but there may be a setting you need to specify to tell it which you are using.
Note the camcorder shows up different in Windows if it is in DV mode than HDV. And in neither case does it show up as a file system in "My computer".
I'll assume you are working in HDV mode... (I.e. you shot HDV, and you don't have the HDV to DV conversion on).
With XP SP2, the driver should be there already. When you connect the camcorder, did you turn it on in "VTR" mode (i.e. play mode, not record mode)?
In your Device Manager, it shows up under "sound, video and game controllers" as "AV/C Tape device". Can you see it there? (See attached screenshot).
If not, you can install it manually by:
Bring up Device Manager, right click on the Microsoft AV/C Tape Subunit, choose Update Driver. Say no to Windows using windows update to find it. Next screen select "install from a specific location". Next screen select "don't search". Next screen, if AV/C Tape device is there, select it, otherwise untick "show compatible hardware". Scroll down the manufacturer list to Sony, then on the right you should see the Sony D-VHS option. Choose that and then try it again.
There is a nice free utility you can also use to capture HDV. I suggest you try this and see if it recognises the camcorder.
http://strony.aster.pl/paviko/
Note it doesn't say it supports the HC3, but it does.