Does Gnome Web browser support the keyword search functionality of Firefox?
So, I define ggi as search google images and type ggi ubuntu in the address-bar to get the image search results?
It has mechanism that I think is superior. You don't use keywords, you just define search sites and they appear as options when you type in the location box.
To set one up, go to the website you want to add and do a search. Use a term you can easily spot later (like "dummy"). Bookmark the site, and if you have a category called "Quick Searches" add it to that. If not, create such a category and add it.
Finally, edit the bookmark by replacing "dummy" with "%s" (no quotes) in the search query. That will be the part of the URL following the "?". There may well be lots of junk there, but you should find your search string somewhere. You might also want edit the title, since many search sites add the term to the title of the returned page.
UPDATE: The manual for Epiphany does not say that the bookmarks need to be in any particular category, and that checks out, despite the manual obviously being rather out of date otherwise. (For example, in 3.6.1 there is no "Show in Toolbar" option).