A recent Duke study investigated the effectiveness of different types of masks in limiting the spread of respiratory droplets.
As others have found, most masks were effective in limiting respiratory droplet spread. However, bandannas were not much better than no mask, and neck gaiters were actually worse than no mask.
Researchers speculate that the neck gaiters broke down larger particles into smaller particles, increasing their spread, similar to a hose nozzle increasing water spray.
The first pic shows the types on mask studies. In green, I marked which ones scored best (1) to worse (14). The second is the data table. (Fischer, E.P., et al. Science Advances, Sept 2020)