Yes, GPLv3 plugins can be released to be loaded in proprietary software. For example, if you make a VST2 plugin intended to be loaded in proprietary DAWs (Cubase, Ableton, FL Studio), you can license and distribute your plugin under the GPL (such as Surge) as long as you sign the VST2 license agreement with Steinberg.
Since Rack plugins are compiled with the Rack SDK and the Rack SDK is open-source (GPLv3), you already have explicit permission to release a GPLv3 plugin, so no need to sign a license like Steinberg requires. In fact, due to the VCV Rack Non-Commercial Plugin License Exception, you have permission to license your plugin under any license of your choice: open-source, freeware, or commercial.
Of course, if VCV combines your plugin into a single software package and released it under a proprietary/commercial license, VCV would be violating your plugin’s GPL license. But if a user combines two packages into a single program for their own use, they have “unlimited permission to run” your plugin (see section 2 of GPLv3 for details). If the GPL didn’t give them this right, it would not be a free software license.