Is it truly the same dependecies set? It looks like it would be better to ipget to have its own deps here. You can try to use go2nix to generate new deps.nix.

I tried and I couldn't do even the step 1 of your instruction really:

% export GOPATH=`pwd`
% go get -d github.com/ipfs/ipget
package gx/ipfs/QmQa2wf1sLFKkjHCVEbna8y5qhdMjL8vtTJSAc48vZGTer/go-ipfs/path: unrecognized import path "gx/ipfs/QmQa2wf1sLFKkjHCVEbna8y5qhdMjL8vtTJSAc48vZGTer/go-ipfs/path" (import path does not begin with hostname)
package gx/ipfs/QmVcLF2CgjQb5BWmYFWsDfxDjbzBfcChfdHRedxeL3dV4K/cli: unrecognized import path "gx/ipfs/QmVcLF2CgjQb5BWmYFWsDfxDjbzBfcChfdHRedxeL3dV4K/cli" (import path does not begin with hostname)
package gx/ipfs/Qmdk8Ea9GkbwHr7UNKVYaLRwwHSt69xBXuSvRVyNWZ9sZE/fallback-ipfs-shell: unrecognized import path "gx/ipfs/Qmdk8Ea9GkbwHr7UNKVYaLRwwHSt69xBXuSvRVyNWZ9sZE/fallback-ipfs-shell" (import path does not begin with hostname)

You see, ipfs projects use their own dependency management stack called gx, which, according to them, is a different thing from what is usually used in go. However it seems like not everything that is needed for build gets downloaded by gx, this is why this needed goDeps attribute as well.

I have a hunch that these missing dependencies are really just dependencies of gx that are somehow leaking through into the ipget build.

So go2nix would need to support gx to be used here.

what's the relation between fetchgx and fetchFromGitHub sources?

There is no relation. As I understand, fetchgx learns about relevant dependencies independently via IPNS and then downloads everything from IPFS. On our side we just put all dependencies in one fixed-hash derivation. I think this is the same way as how rust/cargo in nixpkgs currently work in nixpkgs.