As I said before, there is no way to add data to the start of a file without copying the data at least once. If the file you're modifying has more than one link and you don't want to break the links or the temp file is on a different filesystem than the file you're modifying, you'll have to copy the data twice.

Even if you have more than 20B of memory you can allocate to a program to edit your file, you still have to read the entire 20GB into memory and write the entire 20+GB of data back into your file.

Of course you could create a new filesystem type that allows you to add data to either end of a file without moving existing data and create a new system call to write data into a file at negative offsets. (But, before you ask, I'll warn you that I'm not going to volunteer to design either of those for you for free in a forum like this!)