You would need something which takes an analogue signal as an input and converts it to DV. Most of the products that do that are PC based, i.e. they allow you to record on your PC in DV format.
Here is a box which can work standalone and do it.. but very expensive!
http://www.smartdv.co.uk/product.php/91/0/
A settop DVD recorder with DV-out could record from the TV, then you could take the DV out into the GS300... but as you'd have the recording on the DVD recorder anyway what would be the point?
I guess the question is: why do you want to do this? If you want to edit TV on your PC, then I wouldn't use the GS300 at all, I suggest something like this
http://www.dv247.com/invt/23453