Grammatical category and Part of speech

Grammatical category noun - (grammar) a category of words having the same grammatical properties.

Part of speech and grammatical category are semantically related. In some cases you can replace phrase "Part of speech" with "Grammatical category", this phrases are similar.

Part of speech noun - One of the traditional categories of words intended to reflect their functions in a grammatical context.

Grammatical category and part of speech are semantically related. You can use "Grammatical category" instead a noun phrase "Part of speech".