The entity.gridwithprices.xml.config package gets its data from the aspdnsf_GetProducts sproc, which returns Product.Summary 'out of the box'. Since the data's already there, you should be able to just reference it like any other column in the Product template in that package.
Something like this ought to work to assign it to a variable, then you can just use it wherever you need:
<xsl:variable name="pSummary" select="aspdnsf:GetMLValue(Summary)"></xsl:variable>
(You didn't specify a version so this all assumes 9.5.1, but that functionality hasn't changed much through recent versions)