Connect to netgear cm500v

> I have a Netgear cm500v modem connected through a Sagemcom gateway to
> my computer which is running Windows 10. [...]

   "a Sagemcom gateway" is not a very detailed description of anything.
"connected through" is not a useful way to think of these things.  What,
exactly, is connected to what, exactly, how exactly?  Ethernet cable
from the CM500V modem to "a Sagemcom gateway" WAN/Internet port?
Ethernet cable from "a Sagemcom gateway" LAN port to the (unspecified)
Windows 10 system?

> [...] I also have an old computer which is running Windows 2000
> software which I am trying to connect to the internet through the
> gateway. [...]

   Ethernet cable from "a Sagemcom gateway" LAN port to the
(unspecified) Windows 2000 system?

> [...] I am trying to connect through 'Network and Dial-up Connections'
> but this modem is not in the dropdown list and it is looking for
> manufacturer's files.

   Presumably, your Windows 2000 system is connected to the "a Sagemcom
gateway".  It has nothing to do with the modem; it deals with only the
"a Sagemcom gateway".  You should be trying to configure your Ethernet
interface to your LAN, not anything to do with the modem.  Forget about
the modem.  Only the "a Sagemcom gateway" deals with the modem.  Your
computers deal with the "a Sagemcom gateway".

   I can't remember ever touching a Windows 2000 system, so I can't
tell you how to configure it, but however you do it, the modem will not
be relevant.  What did you need to tell the Windows 10 system about the
modem?  (I assume nothing.)  It should be similar for the Windows 2000
system.