The image is most probably sliced this way to prevent you from exporting it easily as one JPG. See it as some form of copy protection.
If you open the PDF in Acrobat, if you have it, do you also see the grid? If not and you export from Acrobat, will the JPG be OK?
I opened in Acrobat and saved it as a Jpeg then opened it in Photoshop, resized it and saved it for the web. No gaps. Thanks!