In linguistics, a lexical gap refers to a possible word form that just doesn't exist in the language, for any number of possible reasons. This may be due to productive morphology; for example, the word "ungood" is a possible word form in English, but doesn't exist due to the fact that the same meaning is already established in the existing word "bad". (One could argue, though, that a word such as "ungood" might be used in a poetic or emphatic sense.)
A much broader and more liberal sense of the term is applied to possible word forms that fit the phonological patterns of the language, but just don't happen to have any meaning. For example, "norf" could exist as an English word: its sound patterns don't conflict with English phonological rules. It just doesn't happen to have any meaning in English (to my knowledge, anyway.) In contrast, a form such as "ngru" is not a lexical gap in English, as this sequence of sounds would not ever occur as a "natural" word in English.