His given name was probably Henry but he appears to have used Harry interchangeably. Also, Gross appears to be an Anglicisation of his registered surname of Groeser. Harry was active in the Jewish Lads Brigade and the Young Communist League and was the cousin of Louis Stoller of Leeds.
In 1936, he signed up to the International Brigade as hailing from 346, Commercial Rd., in the east end of London, this would seem to be the small sweet shop that his father owned.
When his family discovered he was going to Spain they hid his clothes in a forlorn attempt to prevent him leaving.
Given this, it is especially sad that he was wounded in action on 12th January 1937 and then killed in action on July 16th 1937 at Brunete.
Source: ‘Jews in the Spanish Civil War’