Background The primary cause of HIV transmission is a failure to deliver proper HIV/AIDS information, which is a major public health issue in low- and middle-income nations. While global health coverage continues, there is still little understanding of HIV/AIDS in Ethiopia, and there is a data gap. As a result, the aim of this study was to investigate the factors that influence comprehensive HIV/AIDS knowledge among HIV testing and counselling (HTC) service users in Gondar, northwest Ethiopia. Methods A facility-based cross-sectional study was conducted from June to July 2022. About 423 clients were included. Data on HIV/AIDS knowledge, attitude, and behavior were collected. Stata 17 was used to analyze. A pre-tested questionnaires were used. Using ordinal logistic regression model, variables having p values of 0.2 and <0.05 were chosen as a candidate variable in binary and final model. Results The total knowledge, favorable attitude, and good behaviors of HIV/AIDS were approximately 258 (61%), 220 (52%), and 210 (49.5%), respectively. Being male (AOR = 1.39, 95% CI = 1.28–3.01), age from 31 to 35 years (AOR = 2.13, 95% CI = 1.88–4.18), and above 36 years old (AOR = 2.21, 95% CI = 1.98–3.44), primary (AOR = 1.21, 95% CI = 1.67–3.20), and higher education (AOR = 1.19, 95% CI = 1.03–3.01), employed (AOR = 1.33, 95% CI = 1.09, 3.37), and media exposure (AOR = 2.02, 95% CI = 1.21, 3.61) were factors associated with comprehensive knowledge of HIV/AIDS, respectively. Conclusion The study participants’ knowledge, attitudes, and behaviors regarding HIV/AIDS were rather low. Since HIV/AIDS is still a major public health issue, if the concerned body uses and designs media effectively, encouraging females to attend school, it will have a better chance of combating HIV/AIDS by increasing their knowledge.