No need to pipe to other program to convert the file. You can download from Google Docs in any supported format, by using the existing parameters in the URL address.
https://docs.google.com/document/d/FILE_ID/export?format=FORMAT
where:
FILE_ID is string ID of target file and;
FORMAT is file format of choice i.e. txt
Then, downloading the document from Google Docs as text file is straightforward by using wget or a web browser. Both methods will download the document as text file as expected.
I have tried myself and the output looks something like this:
$ wget https://docs.google.com/document/d/FILE_ID/export?format=txt
--####-##-## ##:##:##-- https://docs.google.com/document/d/FILE_ID/export?format=txt
Resolving docs.google.com (docs.google.com)...
Connecting to docs.google.com (docs.google.com)... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/plain]
Saving to: ‘export?format=txt’
[ <=> ] 649 --.-K/s in 0s
####-##-## ##:##:## (##.# MB/s) - ‘export?format=txt’ saved [649]
The URL address for other products such as Google Sheets, Google Presentation or even Google Drive would be slightly different.
In terms of documentation, the only relevant guide I found was this dated blog post circa 2014. There is this page of developer guide for Google Drive but not useful as it is. That is all.