The gw GGGG argument means that any IP packets sent to this address will be routed through the specified gateway. NOTE: The specified gateway must be reachable first. This usually means that you have to set up a static route to the gateway beforehand.
The metric MMMM modifier is not yet implemented (and with the -v option will actually print a warning).
The netmask NNNN modifier specifies the netmask of the route to be added. This only makes sense for a network route, and when the address XXXX actually makes sense with the specified netmask. If no netmask is given, route guesses it instead, so for most normal setups you won't need to specify a netmask.
The mss NNNN modifier specifies the TCP mss for the route to be added. This is normally used only for fine optimisation of routing setups.
The window NNNN modifier specifies the TCP window for the route to be added. This is typically only used on AX.25 networks and with drivers unable to handle back to back frames.
The dev DDDD modifier forces the route to be associated with the specified device, as the kernel will otherwise try to determine the device on its own (by checking already existing routes and device specifications, and where the route is added to). In most normal networks you won't need this.
If dev DDDD is the last option on the command line, the word dev may be omitted, as it's the default. Otherwise the order of the route modifiers (metric - netmask - gw - dev) doesn't matter.