If your add-on modules are dynamically loaded into GPL-licensed software at runtime, you’ll have to license the add-on modules under the GPL’s terms when distributing them along with the GPL-licensed software; it is a clear-cut case of a “derivative work” under the License. The case is less clear, however, if the add-on module is distributed separately from the GPL-licensed software, as may, for example, happen where the recipient has already installed the GPL-licensed software from a different source.
At first glance, such separate distribution, and, in particular, the acquisition of the add-on module and the GPL-licensed from different sources, suggests that the add-on module in question is not a derivative work of the GPL-licensed software. This seems to be supported by Section 3 subsection 2 GPL-2.0, which excludes the obligation to provide the source code for such software that the receiving party can be expected to already have, even though this software is necessary to run an executable licensed under the terms of the GPL. A counter-exception is then made for cases where the executable was in fact accompanied by the software the receiving party could be expected to already have. Accordingly, a distinction seems to be made between GPL-licensed software and add-ons/additions being distributed alongside one another and the separate distribution, with the first case constituting a derivative work and the latter not.
However, when taking a closer Section 3 subsection 2 GPL-2.0 actually clarifies that GPL-licensed software and an add-on actually form a derivative work, even if they are distributed separately. In other words, artificially separating GPL-licensed software and add-ons to distribute them separately does not change the fact that they form a derivative work. If a separated distribution were to be judged differently than a joint distribution of GPL-licensed software and add-on, an exception like the one made in Section 3 subsection 2 GPL-2.0 would not be necessary.
This view also aligns with the interpretation suggested by the “Frequently Asked Questions about the GNU Licenses” provided by the Free Software Foundation, Inc., according to which a plug-in destined to be used with a GPL-covered program needs to be licensed under the GPL, if the program dynamically links the plug-in.