Gram staining
The adjectives "Gram-positive" and "Gram-negative" derive from the surname of Hans Christian Gram, a Danish bacteriologist; as eponymous adjectives, their initial letter can be either capital "G" or lower-case "g", depending on which style guide (e.g., that of the CDC), if any, governs the document being written. This is further explained at "Gram staining § Orthographic note".