Hello Vitaly
There are essentially three ways to configure a gateway of last resort on an L3 switch or on a router. These differ in their implementation and their functionality.
The ip default-gateway command should only be used when routing is disabled. It essentially tells the device what its default gateway is much like a PC has the default gateway configured. It is used only for the purposes of connectivity with subnets other than its own.
The ip default-network command can be used only when routing is enabled. When you configure ip default-network the router considers routes to that network for installation as the gateway of last resort on the router. In other words, routes to that network become candidate default routes.
A third way of configuring a gateway of last resort is to install a static default route with the command ip route 0.0.0.0 0.0.0.0. Of course, routing must be enabled for this case as well.
You can find more information about these three commands and their uses at this Cisco documentation.
I hope this has been helpful!
Laz