Short overview:
The GPL-licensed code you modified is a web page template, and the code you want to distribute as a web site is the output of rendering that template with some of your own text and images. These are two different works. However, since the template-rending process leaves substantial portions of the GPL-licensed template in the output web page, the web page is a derivative of the template, and it can only be distributed under the GPL as well. (See the second paragraph of this GPL FAQ item.)
Long answer:
The central principles of the GNU GPL are:
If you make a derivative of a GPL-licensed work, then your derivative is required to be under the GPL.
Whenever you distribute a GPL-licensed work (including a modification), the license requires that you make available the human-readable source code to anyone who receives a copy of the work.
Recipients should be aware the work is licensed under the GPL. If you distribute the work verbatim, you must keep notices intact; if you modify the work, you must ensure that notices about the work's license are present.
In your case, the applicable requirements, taken from sections 4, 5, and 6 of the text of the GPLv3 are:
- conspicuously and appropriately publish on each copy [of the software] an appropriate copyright notice
- keep intact all notices of the absence of any warranty
- give all recipients a copy of this License along with the Program
- The work must carry prominent notices stating that you modified it, and giving a relevant date.
- The work must carry prominent notices stating that it is released under this License
- You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply [...] to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.
Finally, if what you distribute is not the preferred form for making modifications, you must also choose a method from section 6 to make the source code available to anyone who receives a copy of your site.
These requirements apply to whatever GPL-licensed material you actually deliver to the recipient viewing your page. If there is GPL-licensed code that constructs your page before it is delivered to the viewer, and no one ever receives a copy of that page-constructing code, you do not need to share that unseen code in any form. However, any GPL-licensed material that the user does receive, such as the HTML, CSS, or JavaScript that the browser renders, must be licensed under the GPL (and appropriately marked) if it is based on GPL-licensed material.
Strictly speaking, the recipients already have received the HTML, CSS, and JavaScript, so you might not need to distribute it separately. However, be aware that:
You still need to ensure the files are marked as being GPL-licensed
You need to make sure the files are not minified or obfuscated, since that is not "source code" as the GPL defines it (specifically, the "preferred form for making modifications")
Depending on the structure of your site, people may find it difficult to gather all the separate files involved in your site -- it would be much easier to provide them as a source-controlled repository or as a downloadable file archive (tar, zip, etc.). Without doing this, a copyright holder could make the argument that your are deliberately impeding people's ability to usefully download the complete source. (It is difficult to say whether or not such an argument would be correct, and may depend on the specifics of how your files are structured and used within the browser client during rendering.)
All these requirements could be satisfied by a simple footer like
© 2018 {my name}
Based on Researcher theme, © 2017 {original author}
Licensed under GNU GPLv3 | download the source here
The "GNU GPLv3" link should be to a locally-hosted copy of the text of the GPL, available from the same server as your site as a whole. The source-download link may point to a remote code-hosting site or a file download where users can download the complete HTML, CSS, and JavaScript of the built site. You do not need to share the Jekyll template, which is never distributed to a viewer of the website.
These requirements are not affected by whether your distribution is commercial or noncommercial.
At this point, you might be saying, "Wow, do I really need to license my entire site under the GPL just because the template is under the GPL? That doesn't seem great." In fact, the Free Software Foundation agrees with you, and explicitly discourages the use of copyleft licenses for website templates:
Templates are minor enough that it is not worth using copyleft to protect them. It is normally harmless to use copyleft on minor works, but templates are a special case, because they are combined with data provided by users of the application and the combination is distributed. So, we recommend that you license your templates under simple permissive terms.
So, the author of the Researcher template chose to license their template under the GPL, in contravention of the FSF's advice. However, since the original author did choose the GPL for their template, you must deal with the consequences: namely, your entire site that is the built from this template must be licensed under the GPL to avoid a license violation.
