Do you prefer code or D/D (Drag and Drop)? If you ask me, GML is easier than Drag and Drop but then ..
An example is ..
//Press Left Key
sprite_index=spr_RunL
//Release Left Key
sprite_index=spr_Stand
//Press Right Key
sprite_index=spr_RunR
//Release Right Key
sprite_index=spr_Stand
Yeah, you'll need a bit of tweaking to get the code to work properly with your game. It's simple and I know there are some flaws, but it should still be useful.