I don't think you need that parameter, you can simply use XPath to access the ancestor element (grandparent) and its Activity child element:
Code:
<gmr:Cell Row="{$vim2}" Col="1" ValueType = "60"><xsl:value-of select="parent::Products/parent::DiscussedProduct/ActivityId"/> </gmr:Cell>
If you want to use a parameter then you either need to use XSLT 2.0 and tunnel it through to the template for the grandchild or in XSLT 1.0 you need to add a template for Products that passes the parameter on explicitly. Currently you pass the parameter on in the template for the grandparent and want to receive it in a grandchild, that does not work.