Is there any quick way in Python to get the MAC address of the default gateway? 
I can't make any ARP requests from the Linux machine I'm running my code on, so it has to come directly from the ARP table.
You can read from /proc/net/arp and parse the content, that will give you couples of known IP-MAC addresses.
The gateway is probably known at all times, if not you should ping it, and an ARP request will be automatically generated.
You can find the default gw in /proc/net/route
Python: get MAC address of default gateway