currently sys-kernel/gentoo-sources-4.20.5 tries to pull patch-4.20.5.xz but its verifying checksum has failed constantly, Manifest for sys-kernel/gentoo-sources seems to be broken. please check and update to fix this issue.
Possibly it's a timing issue when the patch gets to the mirrors. There's a short period where the ebuild could be available before the patchset. No one in #gentoo-kernel reported any problems and two of us retried it ourselves without issue. Can you try again ?
This ebuild is no longer in the tree