While it is definitely possible to reverse-engineer and modify the software/firmware of existing proprietary GPS systems, I'd argue that the distinction between this and changing an open source project is not meaningless.
Changing a couple lines of well-documented source code in an open source project before compiling is arguably a much lower bar to pass.
It depends. For most reasonable firmware, trying to figure out how to compile the stupid thing is generally harder than finding and byte patching a condition in a binary blob.
To someone willing to spend more than a couple minutes, the fact that it's open source is quite meaningless.