*.g70 files can be 1 of two things.
1) A YoYoGames Sandbox legacy Instant Play Web Browser Plugin game.
2) The game file for a GM4Mac 7.5 file found in the app bundle and the GM4Mac runner executable reads this file to load and interpret byte code storing your GML and other resources such as images and audio from your sprite, background, and sound resources.
As such, unless you uploaded you game ages ago to the sandbox and it was converted to an instant play game, or you converted your game to a Mac App Bundle with GM4Mac, you shouldn't have any file with that file extension. Leads me to believe you want to know how to decompile it and it isn't really your game but i file you found on your computer hoping it contained the necessary data for decompilation. GM4Mac games never had a decompiler released, instant play games have however. Depending on what the file is formatted as, (GM4Mac or InstantPlay bytecode) you are bound to make the creator that game very unhappy and lose all your legal rights for using GameMaker as well as YoYo revoking your licenses, whether purchased legitimately or pirated.
I'll hope none of this is the truly case, and you are innocently wondering why you have a file of the same name as your project but with a different file extension. But as I stated earlier, that file extension can only be one of two possible things that I'm aware of, and if it isn't one of those things, it likely isn't your game and you are looking to steal someone's work and claim it as your own.
Essentially a *.g70 file is an outdated format of the current game.ios found in a GMS 1.x or GMS 2.x mac app bundle, which is the same exact file storing the same exact data as the current data.win file shipped with VM Windows runners "compiled" (using that term very loosely as nothing is actually being compiled to a binary executable) with GMS 1.x or GMS 2.x. An instant play *.g70 file contains a Windows icon *.ico file and a GM4Mac *.g70 contains no icon data to my knowledge as that is stored in the bundle seperately. The reason an Instant Play *.g70 file is formatted differently than a GM4Mac *.g70 is probably due to the presence of an *.ico vs the lack there of, and the two are probably using different compression and encryption methods, but honeslty, I have no idea as this is just speculation and I just find binary blobs and interesting topic.
Not much has changed with GameMaker's architecture since the Pre-Studio days, has it? These are things they don't tell you. The real change was from Delphi to C++ and Objective C, which Delphi wraps and as a result is slower. But the reality is, GameMaker was written originally by Mark Overmars using a runner or "rundata" portion of an executable and the bytecode portion. That hasn't changed to this very day, the only difference now is C++ and bytecode can be tacked on to that rundata with LLVM (YYC) and the VM runner is just "the latest version" of the same technology skeleton that has been around for more than 2 decades, at GameMaker's roots.
<don't kill me>