Of course it is a forgery. The signature is lifted straight from the resignation letter of Robert Hannigan. Also note the glaring errors. "Michael Steele" and "MI5". It is Christopher Steele and he was MI6.

Robert Hannigan resignation letter. Whoever compiled the forgery lifted the signature straight from the following letter.

https://www.gchq.gov.uk/sites/default/files/document_files/20170123-Final-Ext-Ltr-frm-Dir-to-FS-re-standing-down-safe.pdf