"ip default-gateway" command works. It is executed from the "Global Configuration" mode.
Output from Packet Tracer version 8.2
Switch#show version
Switch Ports Model SW Version SW Image
------ ----- ----- ---------- ----------
* 1 26 WS-C3560-24PS 12.2(37)SE1 C3560-ADVIPSERVICESK
Configuration register is 0xF
Switch#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#ip default-gateway 192.168.1.1
Switch(config)#end
Switch#
%SYS-5-CONFIG_I: Configured from console by console
Switch#show running-config | include default-gateway
ip default-gateway 192.168.1.1
Switch#
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