Industrial classification systems classify economic activities in countries and regions. There are several different classification systems applied to industries and products, and different information sources will use different classification systems to define industries and compile statistics about it.
In North America, the most common systems are Standard Industrial Classification, which has a US version and a Canadian version, and North American Industrial Classification System which also has some variations for Canadian application. In Canada and the US, the NAICS has succeeded the SIC, but the SIC is in use in other countries (for example, the UK)
Classification manuals provide a description of the industry/products for each classification number. This can be helpful in finding information in various sources that use the classification systems for compiling and publishing industry data. You can find industry classification manuals in the Library Catalogue by doing a Keyword search for industries and classification in the Subject field.
The Federal Reserve economist Alison Weingarden has created a crosswalk/conversion file to go between GICS and NAICS -- see https://sites.google.com/site/alisonweingarden/links/industries
Then the researcher can go from NAICS to ISIC. See for example – https://www.census.gov/eos/www/naics/concordances/concordances.html
It seems there are no conversion charts that go directly from GICS to ISIC, but use the above workaround.