Grammatical marking is a cover term for marking by (i) °grammatical markers, i.e. bound non-root forms (especially affixes, but also "clitics" of various kinds), (ii) °reduplication, or (iii) endophony.
A quasi-synonym of "marking" is "coding", and "grammatical" is redundant because "marking" implies "grammatical" (there is no such thing as "lexical marking"; however, "lexical coding" is sometimes used as a synonym of "lexical expression"). "Grammatical marking" is broader than "use of a grammatical marker", because grammatical marking may also occur by root modification or reduplication (e.g. English past tense may be expressed by root modification: sing vs. sang).