As others have already noted, make sure that for every
deb … entry in
/etc/apt/sources.list.d/*, you have a matching
deb-src … entry. The rest of the line must be identical.
deb entry is for binary packages (i.e. ready to install), the
deb-src is for source packages (i.e. ready to compile). The reason why the two kinds of packages are separated is that they are managed very differently: binary packages have a dependency tracking mechanism and a currently-installed list, whereas source packages are only tracked so that they can be downloaded conveniently.
Note that when discussing package repositories, the word source means two unrelated things: a source as in a location to download packages from, and a source package as opposed to a binary package.
libgtk2.0-0 is the name of a binary package. It is built from a source package called
gtk+2.0. The reason source and binary package names don't always match is that building a source package can produce multiple binary packages; for example,
gtk+2.0 is the source for 14 packages as it is split into two libraries (
libgail), corresponding packages to build programs using these libraries (
…-dev), documentation for developers (
…-doc), companion programs (
You can see the name of the source package corresponding to a binary package by checking the
Source: … line in the output of
dpkg -s BINARY_PACKAGE_NAME (if the package is installed) or
apt-cache show BINARY_PACKAGE_NAME.
You can list the binary packages produced by a source package with
aptitude search '?source-package(^SOURCE_PACKAGE_NAME$).
apt-get source downloads a source package. If you give it an argument which isn't a known source package, it looks it up in the database of installable binary packages and tries to download the corresponding source package.
apt-get build-dep follows the same approach to deduce the name of a source package, then queries the source package database to obtain a list of binary packages (the list in the
Build-Dep: field), and installs those binary packages.
The Software Sources GUI has a checkbox “enable repositories with source code” for official repositories, make sure that it's ticked. If you add third-party repositories manually, make sure that you add both