“Who is Abimelech, and who are we of Shechem that we should serve him?” The emphasis here is “WHO is Abimelech, and WHO ARE WE of Shechem, that WE should serve HIM (HE should be serving US!).
“Serve the men of Hamor the father of Shechem!” The verb “serve” is an imperative, and Hamor the father of Shechem goes back to the time of Jacob, which pre-dates the founding of Israel. Whether or not the men of Shechem were Israelites who moved there or a mix of Israelites and native Canaanites, they were worshiping the local god Baal-berith and identified themselves more with Hamor, an early ruler of Shechem than with the Israelites who came into the land from Egypt. So Gaal is telling them to not serve Gideon’s son Abimelech but to serve him, he apparently being a native of Shechem.
“Why should we serve him.” The rhetorical question gets people to say, “Yes, why indeed?” and rebel against Abimelech.