All the code you can edit is in the Code tab.
There you can find alot of different scripts that have different functions.
For testing purposes open a random file.
There are 3 tabs Disassembly, Graph View(external software require and I got no clue wtf it does) and Decompiled.
In the Disassembly tab you see the code that you can edit.
And in the decompiled tab you see neat gml(GameMakerLanguege) script (I think)
Alot of the stuff about items happen in script called "gml_Object_obj_master_controller_Other_10"
It's damn long, and trying to open it as decompiled crashes my pc :C
But if you scroll a little and find items, you can change their properties.
For example you can take line like this:
31607: pushloc.v local.object
31609: pushglb.v global.itemHealth
31611: call.i ds_map_add(argc=3)
31614: pushi.e 50
And change the 50 (healthValue) to 200000 and you got block(weapon in this case) that can't realy be destroyed
There are commands in-game that can give you custom items, but if you do that.... it's basicly cheating....
Keep in mind that also your enemies have this block and some fights can get pretty long.