I wish to restore a gzipped tarball - what does mongodump --archive do? Howto do a mongorestore with --gzip?
I think the problem you have is that the archive format created with --archive is not a tarball (and the docs don't say it is anywhere that I could find). Rather it is a custom packaging format which you can see the details of here. Based on a quick scan of the code, it looks like a lightweight format containing a series of headers, metadata which describes the raw BSON. Short of creating a standalone binary to create a compatible archive file, you can't do this manually.

If you do mongodump without archive, but with --gzip, it will actually gzip the individual files, and you can emulate that by doing the dump normally then gzipping each file in the folder separately. Those compressed files could then be restored theoretically with mongorestore --gzip.

Overall I would advise to just use --archive in the 3.2 tools and stay away from trying to recreate it manually, but the --gzip option is as close as it is going to get without a bunch of work.