There are several products on the market. Gunk is but one of them. Best?? I haven't tried them all, but find Gunk works for me.
Scraping the bulk of it off with a putty knife is what I do, as a lot of it is dirt mixed with oil.
Then spray on the Gunk-like product, hose off with hot water (or cold if hot not available), and do a second application if not satisfied with the first.
Plug up any entry points where you don't want water to get in first.
We like pics, especially before and after ones.