If a file isn't secret, but you still need to ensure authenticity and integrity, you can just sign it without encrypting it:

donnie@ubuntu:~$ gpg -s not_secret_for_frank.txt

You need a passphrase to unlock the secret key for
user: "Donald A. Tevault <donniet@something.net>"
2048-bit RSA key, ID 9FD7014B, created 2017-10-27

gpg: gpg-agent is not available in this session
donnie@ubuntu:~$ ls -l
. . .
-rw-rw-r-- 1 donnie donnie  40 Oct 27 19:30 not_secret_for_frank.txt
-rw-rw-r-- 1 donnie donnie 381 Oct 27 19:31 not_secret_for_frank.txt.gpg

Just as before, I create a .gpg version of the file. When Frank receives the file, he may try to open it with less:

frank@ubuntu:~$ less not_secret_for_frank.txt.gpg
"not_secret_for_frank.txt.gpg" may be a binary file.  See it anyway?

<A3>^A^Av^A<89><FE><90>^M^C^@^B^A<88>o3W<9F><D7>^AK^A<AC>Fb^Xnot_secret_for_frank.txtY<F3><C1><C0>This isn't secret, so I just signed it.
<89>^A^\^D^@^A^B^@^F^E^BY<F3><C1><C0>^@
        ^P<88>o3W<9F><D7>^AK6<AF>^G<FF>Bs<9A>^Lc^@<E9><ED><C2>-2<AE><A7><DF>ͺaB
<EC>/[:<D1>{<B2><FD>o8<C6><C9>x<FE>*4^D<CD>^G^O^F<F3>@v<87>_1<D0>^Bp<FE>q^N3<B0><BE><85><D2>9]{<D6><EF><9A><D8>  `<C2><E4>^NC<9B>        "Ձ^?M<89>s<9F>z^B"<DF>
s}`<A4>:<B4>&<F7><F4>\EjȰ!^Q <9C>6^E|H<E2>ESC<D9>9<DC>p_ӞESCB<DE>^P<FF>i<CA>)^O
<A0>     <CB><C4>+<81><F5><A7>`5<90><BF>Y<DE><FF><<A0>z<BC><BD>5<C5><E8><FE>>
<B7><A2>^L^_^D<DD>Kk<E0><9A>8<C6>S^E<D0>fjz<B2>&G<A4><A8>^Lg$8Q>{<FF><FA>^M_A
<A1><93><C3>4<DC><C4>x<86><D9>^]- <8A> F0<87><8A><94>%A<96><DF><CD>C><80><C3>l
<D3>K<E5>^G<8E><90>d<8C><DA>Aɱb<86><89><DA>S<B6><91><D8><D2><E0><B3>K<FC><9E>
<ED>^@*<EF>x<E7>jø<FD><D3><FA><9A>^]
not_secret_for_frank.txt.gpg (END)

There's a lot of gibberish there because of the signature, but if you look carefully, you'll see the plain, unencrypted message. Frank will use gpg with the --verify option to verify that the signature really does belong to me:

frank@ubuntu:~$ gpg --verify not_secret_for_frank.txt.gpg
gpg: Signature made Fri 27 Oct 2017 07:31:12 PM EDT using RSA key ID 9FD7014B
gpg: Good signature from "Donald A. Tevault <donniet@something.net>"
frank@ubuntu:~$