How to make 2 subnets share 1 default gateway on a Cisco switch or router?

This is not possible to have common default gate for different Subnets.

using private VLANs will make this to happen.

you can use only one subnet and divide them with two VLANs and set the port connected to the default as promiscous port by using private vlans

By segmenting your LAN into Vlan then configure trunk port which connects to the core switch, Its good in Educational enviroment or where you want diffrent users access diffrent resources with the LAN

by intervaln to make interface with two service  and make one virtual gateway 

Gateway should be from same subnet.I

if you divide a subnet into two vlans that will again be two different subnets.