ROI is (net benefit)/(cost), where net benefit is (gross benefit - cost). Net benefit can be negative, making ROI negative if cost exceeds benefit. GP was using benefit as a shorthand for net benefit, which can certainly be negative.

Even gross benefit could conceivably be negative, if the ads are bad enough! (I.e., a terrible ad might prevent a sale.)

We tend to call most rations below 1 'negative'. Its technically wrong but most people get what you mean when you say it.

Positive and negative are the right words on a log scale :D

Ratio can't be negative, but ROI can. Positive ROI is the percentage of the ratio being 1 or higher. Negative ROI is the percentage of the ratio being less than 1.

Or, from a "common sense" perspective, if I spent $1000 on advertising, and earned $100 in revenue from that investment, my ROI is definitely negative.