Sure... so here's why I think it's poorly designed:

• The text spans across the entire page, making it harder to read. This is not good if you're designing a document that is supposed to be scanned quickly. The recommended line length is 50-60 characters per line. Gayle's resume template averages about 100 characters. See http://baymard.com/blog/line-length-readability
• The margins are too tight. There isn't enough space between content, and it makes the resume feel cluttered. Scanning the resume is difficult. See rule 3 on https://medium.com/@erikdkennedy/7-rules-for-creating-gorgeous-ui-part-1-559d4e805cda
• The design doesn't do a good job of exhibiting hierarchy. It should use bigger font sizes for headings to draw the reader's attention. It feels like all of the content is shouted at the reader at once, making it hard to know where to look. A design should have a hierarchy that is easy to discern. The horizontal lines help, but they're also really tacky. Horizontal lines, if used, should be lighter in color so that they don't stand out like a sore thumb.
• The formatting isn't great. Some of the date ranges use en dashes as delimiters and some of the date ranges use hyphens. Some of the lines end with periods and others don't. Some of the years are bold and others are not. Consistency is crucial if you want a crisp user experience.
• The resume uses centered text. Centered text is generally something that you should avoid. Really good designers can use centered text well, but Gayle is not one of those designers. See rule 7 on http://www.webdesignerdepot.com/2009/02/10-web-typography-rules-every-designer-should-know-2/.
• The typography is bland (at least it's consistent) and too small. Most resume fonts are bland, and that's understandable. However, well designed resumes use classier fonts. A larger font size will also improve readability. I'm also not a fan of using small caps for sans serif text: it looks forced and out of place. See rule 6 on https://medium.com/@erikdkennedy/7-rules-for-creating-gorgeous-ui-part-1-559d4e805cda and http://practicaltypography.com/small-caps.html.
• It's hard to scan. If I'm looking for dates, sometimes they are on the right, and sometimes they are on the left.

Note, in our field, bad design probably isn't going to prevent you from landing a job. However, good design is never a bad thing :)