Using Garry's Mod via Steam on iMac

I'm 61. My 5 year old grandson plays Garry's Mod via Steam. Garry's Mod no longer opens on our Mac. I already went in to Properties and Betas and changed it to 64b or whatever I was told to do, it worked for a while but just quit working. I have uninstalled and reinstalled Steam but still can't get Garry's Mod to open. Any suggestions??