Nutrient16 recently put together an Injustice: Gods Among Us tutorial that details how to use Lex Luthor’s gravity mine move to set up an unblockable trap on push block. The trap is accomplished by push blocking an opponent back to an already placed gravity mine. If done at the right distance, the opposing character will still be in a recovery state, meaning they will be unable to block the gravity mine when they hit it.
The video covers how Lex players can use this trap as well as ways they can counter it if another Lex tries to use it against them.
Source: Nutrient16, thanks to link3kokiri for the tip