This code snippet will format field id 6 on form 2 as a date field in excel

gravityexport_format_date_excel.php

<?php // DO NOT COPY THIS LINE

$binder = new \PhpOffice\PhpSpreadsheet\Cell\AdvancedValueBinder();
add_filter( 'gfexcel_renderer_cell_properties_2', function ( \PhpOffice\PhpSpreadsheet\Cell\Cell $cell, \GFExcel\Values\BaseValue $value ) use ( $binder ) {
	if ( $value->getFieldId() == 6 && $cell->getRow() > 1 ) {
		$old_binder = \PhpOffice\PhpSpreadsheet\Cell\Cell::getValueBinder();
		\PhpOffice\PhpSpreadsheet\Cell\Cell::setValueBinder( $binder );

		$cell->setValue( $value->getValue() );
		$cell
			->getStyle()
			->getNumberFormat()
			->setFormatCode( \PhpOffice\PhpSpreadsheet\Style\NumberFormat::FORMAT_DATE_YYYYMMDD );
		\PhpOffice\PhpSpreadsheet\Cell\Cell::setValueBinder( $old_binder );
	}
}, 10, 2 );

Replace the form ID in the filter name: gfexcel_renderer_cell_properties_2 the 2 at the end here is meant to target the form ID = 2.

The '6' here on line 5 is to target the date field if ( $value->getFieldId() == 6

Here's where to add this code: https://docs.gravitykit.com/article/210-where-to-put-code-samples