Final answer:
The gizmo industry, with its many producers, differentiated products, and no long-term economic profit, is categorized as a monopolistically competitive market.
Step-by-step explanation:
The gizmo industry is best described as a monopolistically competitive market. This type of market is characterized by many producers, none of which have a significant market share, and differentiated products with unique brand names and labels. Additionally, the lack of significant barriers to entry and the absence of economic profit in the long run are typical of monopolistic competition.
Given the characteristics you described, such as about 50 producers with no one producer furnishing more than 5% of the industry's total output, unique brand names and labels, no significant barriers to entry, and no economic profit in the long run, this assessment fits the gizmo market. It differs from perfect competition in that products are not homogeneous, and from oligopoly due to the larger number of firms and the absence of dominant players.