Hi,
Garage band indeed loads only Stereo components of Waves/ other 3rd party plugins and runs mono inputs through them.
Since some of Waves plugins has only a Mono component (DeBreath/ PRS for example), in order to insert a mono-component onto a track, try the following:
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Open a new track with a mono input - it will load with two default mono plugins. Compressor and EQ.
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Replace the Compressor with any mono component.
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Drag the mono plugin to the desired insert slot.