Doesn't seem like it's just one source code file if op states : "Requirements are: size < 1GB,"

Depending on font size (say 10pt) and average characters per line, that would be printing several hundred thousand paper pages which is not feasible for the average homeowner to roundtrip back into usable digital files.

Instead of a cheap flatbed scanner, you now need a high-speed auto-fed document scanner and then run batch jobs to OCR several hundred thousand tif images back to digital source files.

One could reduce the paper count by compressing the source text to zip and then printing a form of binary-to-text (e.g. UUENCODE) but now the papers have random-looking gibberish instead of readable text.

Printing <10 MB source code is more realistic than <1 GB.

(But I'm guessing the author may actually has much less than 1,000,000,000 bytes of original source code if one leaves out 3rd-party dependencies.)