Request #3910 has been rejected by alucryd [1]: Wrong, both only build libgnutls.so.28 and not libgnutlsxx.so.28, hence lib32-libgnutls28 is the correct name. lib32-gnutls28 should provide all libs that the regular gnutls package provides.
[1] https://aur.archlinux.org/account/alucryd/
