"Gradual deterioration" means a slow and steady decline or worsening of something over time. It suggests that the condition or quality of something is gradually getting worse, rather than deteriorating suddenly or rapidly. Full definition
I've seen the more than gradual deterioration of good business ethics and the treatment of others in our business and it bothers me. (remonline.com)
Industrial mass produced refined vegetable oils have been introduced into use after WW2 with consequential gradual deterioration in public health, and huge increase in the obesity rate world wide. (fathead-movie.com)
In contrast, when the market is free from the hostile syndrome we observe here, it's often possible to get some amount of «warning» about potential trouble from gradual deterioration of market action and a tendency for various «divergences» to develop as risk aversion increases. (hussmanfunds.com)