Whenever I try to download PoE I get a message saying I don't have enough disk space. But I have over 40GBs open and PoE is only 7 GBs. Is this some kind of bug? Is there any way to fix it? I'm downloading it using the gog client

there are few things that can explain this behaviour.
1. local time machine backup. look under "apple->information for this mac->Archive" (I tried to translate from italian so it could be different) how much space you really have on your mac. If it is different from the one you see from finder you probably need to do a time machine backup or temporaly deactivate it to clean the local backup.

2. gog galaxy download the full uncompressed version when you try to install a game. if you try to download the installer from backup & goodies you should get a much smaller version of the game (when you install it of course you need more space, but usually it is easier to download smaller things and uncompress them locally, also you can decide to install the game on an external drive if you use the installer, gog galaxy only use the internal drive).
PoE is not 7GBs, it is 15GBs for the base game (I got it on steam because it was a gift (fortunately it is one of those game that you can play without steam installed), I still have the original base game on my external drive and it is 15GBs). Maybe the installer is 7GBs but galaxy don't download the installer if you click on Install.