Perhaps I missed something though? (https://www.youtube.com/watch?v=at72dhg-SZY&feature=youtu.be...)
First, TUF allows you to have freshness guarantees over the content. In GPG's model a MITM or malicious mirror can serve you old, known vulnerable content that you'll accept as valid because the signatures verify. This is not possible with TUF as metadata is additionally signed with a timestamping key.
Second, TUF has a property called 'survivable key compromise' which basically means that there are a hierarchy of keys involved in the system, each with a different responsibility and security requirements. There's a root key that's kept offline, a target key responsible for signing actual content, a timestamping key for freshness, and a snapshot key to tie all the other keys together. GPG's model does allow for signing subkeys, but it is rather clunky to use and many of the Linux package managers don't support using signing subkeys, sadly.
Finally, GPG's usability leaves something to be desired. Docker makes pushing and pulling of images extremely easy, essentially making everyone a publisher of content. GPG works when publishing software is more rare and you can take the time to use a new utility in order to get security guarantees, but we wanted to make it extremely easy so that anyone can do it.
For more background, this paper does a good survey of existing package managers and where they fall short: https://isis.poly.edu/~jcappos/papers/cappos_pmsec_tr08-02.p...
This presumes you use HTTP, have a compromised SSL cert, or have pissed off the NSA. At the worst one would be installing older packages with known vulnerabilities via a replay attack, not a fresh code injection by the attacker. This is more the fault of the package manager running over HTTP than GPG, as far as I can see.
Here's a paper covering 'survivable key compromise' by the same chaps: http://freehaven.net/~arma/tuf-ccs2010.pdf. Interesting stuff.
This is not taking into content mirroring. TUF allows you to treat all mirrors as potentially malicious allowing anyone to reliably deliver trusted content, even in an untrusted network. GPG does not provide a way to detect active attacks other than signature verification.
A wrapper would have done an awesome job at that... I use GnuPG daily, enter a passphrase once and boom. Mails are signed, my password manager unlocked. Where is the "unusability" in this?
TUF should be understood as a higher level concept than GPG. There are additional features of the TUF spec that we'll be implementing in later versions, such as threshold signing (k of n signatures required for verification) and secure delegation.
For what its worth, TUF could be implemented on top of GPG just fine. If folks have an appetite for that we'd welcome contributions here: https://github.com/docker/notary
Python is also looking at using this for their package management system, pip.
[1] Discussion: https://news.ycombinator.com/item?id=9708120