As of now it seems the only way to edit the automatic hyperlinked heading added to the top of every page is through custom CSS. There is no way otherwise to change font size or family and you are forced to use the preset size of the theme. In other blocks it at least allows you to change between the various headings/paragraph.
Screenshot for reference:

Seen at http://alittletestblog.com/wp-admin/post.php?post=14188&action=edit running WordPress 4.9.8 and Gutenberg 3.4.0 using Firefox 61.0.1 on macOS 10.13.6.
You were not able to edit the styling of the post title in the Classic Editor, either, so nothing has really changed there.
Currently, Gutenberg only edits post_content, or in less technical terms: the main content of a post/page. The title of a post is actually outside of this content area, as are things like the post meta info (text listing the publish date, author, etc.), comments section, featured image, and other things like that.
Notably, changing those areas affects all posts, so editing that area falls into page template building, which is not something Gutenberg is covering yet. It is planned for Gutenberg to cover those areas in the future, as well as other areas like sidebars and headers and footers, but not until _after_ WordPress 5.0.
Currently, the Gutenberg project is focusing on trying to get the post editor aspect done right; it is trying to simply be a good replacement for the Classic Editor, with some foundational block APIs (nested blocks, block templates, block style variations like the ones used by the Quote block) set in place for future enhancements. The need for an easy way to test nested blocks is the only reason why there is a Columns block in Gutenberg right now; if it weren't for that, a Columns block would not be added to core until later. Gutenberg is not trying to be a page builder yet. It is trying to be a block-based editor. Most page builder plugins are pretty bad at just being a simple post editor, since they were designed with page building as the primary focus. By focusing on getting the text editor experience right, Gutenberg will be better equipped to become a full-fledged page builder and later website builder in the future.
So anyway, I would not consider it a bug that you can not edit the styling of the title of a single post. You were not able to in the Classic Editor, and Gutenberg is not even covering post templates yet, so I would consider this a non-issue.
@ZebulanStanphill I completely understand the Gutenberg focus on post_content side but I am writing this comment because a client of mine actually had the same kind of feeling.
She wanted to change the alignement of the title because, different from the classic editor where the two section were well separated, Gutenberg does not show any separation between the title and the actual post content.
I think that would be useful to add a kind of inline help or something like it in the "welcome tooltips" (I do not remember the actual name of the tooltips shown the first time a user uses Gutenberg) that explain that the user can change the styles of all the content but not the title of the article.
@AndreaBarghigiani Yeah, I think it would be a good idea to have more visual separation between the title and the block editor, because right now it looks like the title is just another block, which is not actually the case.
This issue seems to be a duplicate of #8469, so I'm closing it in favour of that one. See also #11553 for the likely future of post title as a block.
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@AndreaBarghigiani Yeah, I think it would be a good idea to have more visual separation between the title and the block editor, because right now it looks like the title is just another block, which is not actually the case.