To collect information in small amounts and often with difficulty. (Cambridge Dictionary)
Obtain (information) from various sources, often with difficulty. (Oxford Dictionaries)
Collect gradually. (Oxford Dictionaries)
If you glean something such as information or knowledge, you learn or collect it slowly and patiently, and perhaps indirectly. (Collins Dictionary)
To gather grain or other produce left by reapers. (Merriam – Webster)
To gather information or material bit by bit. (Merriam – Webster)
To glean or not to glean? Why you shouldn’t let bad fruit hang around.