I use both of those applications: Shareaza on windows and gtk-gnutella on linux. However, I never had toubles downloading from gtk-gnutella in Shareaza, and neither downloading from Shareaza in gtk-gnutella. Both works fine for me.

Shareaza does not ban gtk-gnutella, because it has no reason to do so and it wouldn't be fair. The reason that you can't download off some Shareaza clients is the following: Shareaza can connect to multiple networks and many people only use some of the networks that Shareaza supports. This means that some Shareaza users will not enable the Gnutella network, but use some other network instead, like eDonkey2000, G2 or BitTorrent. And Shareaza can be configured to not upload or download from networks that are not being connected to. Therefore, if you use a Gnutella client like gtk-gnutella, you can't download from a Shareaza client that has Gnutella disabled and that doesn't allow uploads to networks that aren't connected. But that's only fair, because those Shareaza clients won't download from Gnutella either and therefore won't download from gtk-gnutella.

Shareaza has always used the same policy when deciding which clients to upload to and which clients to download from. When Shareaza downloads from client X, it will also upload to X. (On a per-user basis of course.)