There is an open-source compiler called GALasm at Github.

GALasm

Pros:

• Open source
• Runs on Linux natively

Cons:

• Limited to a few GAL chips
• Needs Opal files to be edited to a new format
• Pin lists must include NC pin names
• Pin names limited to 8 characters

Opal

Pros: 

• I can run it on MSDOS and a Linux DOS emulator
• Supports many GAL and PAL chips
• Written by a chip manufacturer
• Thoroughly tested by industry

I shall be sticking to Opal, because I can run it on both machines