Similiar question asked here
How to convert Google spreadsheet's worksheet string id to integer index (GID)?
BUT!
I think the Google Drive API was changed recently and now the spreadsheets, created as Form Responses, have different GIDs, for example 891536317 and the string equivalent is "oeqspk7".
The algorithm from before mentioned post doesn't work here. Please read it if you don't know what I'm talking about here.
It seems it is also base36 encoded but maybe with different shift?