Since configurator is base on flex. so you can use flex css and atrribute in the .gpc file.
Below is a example about how to modify the header height of the accordion.
<ConfiguratorPanel>
<accordion _itemId="container_accordion" headerHeight="100" toolTip="$$$/Configurator/Container/Accordion/desc" label="$$$/Configurator/key_###1" width="100%" height="100%" cfgVisible="$$$/Configurator/Attribute/visible/value###2" x="20" y="20">
<canvas _itemId="container_canvas" label="$$$/Configurator/Accordion/Canvas###3" width="100%" height="100%" toolTip="$$$/Configurator/Container/Canvas/desc" cfgVisible="$$$/Configurator/Attribute/visible/value###4"/>
</accordion>
</ConfiguratorPanel>
<elements>
<element id="accordion" classRT="com.adobe.configurator2.component.AccordionRuntime">
<attr id="headerHeight" meta="UI_STYLE" localize="n" translate="n"/>
<attr id="label" meta="UI_ACCESSOR" localize="y" translate="y"/>
<attr id="toolTip" meta="UI_ACCESSOR" localize="y" translate="y"/>
<attr id="top" meta="UI_STYLE" translate="y"/>
<attr id="bottom" meta="UI_STYLE" translate="y"/>
<attr id="left" meta="UI_STYLE" translate="y"/>
<attr id="right" meta="UI_STYLE" translate="y"/>
<attr id="horizontalCenter" meta="UI_STYLE" translate="y"/>
<attr id="verticalCenter" meta="UI_STYLE" translate="y"/>
<attr id="x" meta="UI_ACCESSOR" translate="y"/>
<attr id="y" meta="UI_ACCESSOR" translate="y"/>
<attr id="cfgVisible" meta="UI_ACCESSOR" localize="y"/>
<attr id="width" meta="UI_ACCESSOR" translate="y"/>
<attr id="height" meta="UI_ACCESSOR" translate="y"/>
</element>
</elements>