Gamma-marking

In generative syntax, gamma-marking is a mechanism which assigns the feature [+gamma] to empty categories that are properly governed. For chains terminating in an argument position, it is assumed that gamma-marking takes place at S-structure, while for chains terminating in a non-argument position it is assumed to take place at LF. Empty categories that are not properly governed receive the feature [-gamma] by default. In this way, applying the ECP means checking whether an empty category has the feature [+gamma] at LF.

Link
Utrecht Lexicon of Linguistics