Verse 39:[1] And Gaal went out before the men of Shechem, and fought with Abimelech.
[Therefore, Gaal went out] He fights, but perhaps less prepared: for Abimelech had arrived unexpectedly (Martyr).
Verse 40:[2] And Abimelech chased him, and he fled before him, and many were overthrown and wounded, even unto the entering of the gate.
He fled before him; being surprised by the unexpected coming of Abimelech, and possibly not fully prepared for the encounter. Many were overthrown and wounded, being pursued and overtaken by Abimelech.
[1] Hebrew: וַיֵּ֣צֵא גַ֔עַל לִפְנֵ֖י בַּעֲלֵ֣י שְׁכֶ֑ם וַיִּלָּ֖חֶם בַּאֲבִימֶֽלֶךְ׃
[2] Hebrew: וַיִּרְדְּפֵ֣הוּ אֲבִימֶ֔לֶךְ וַיָּ֖נָס מִפָּנָ֑יו וַֽיִּפְּל֛וּ חֲלָלִ֥ים רַבִּ֖ים עַד־פֶּ֥תַח הַשָּֽׁעַר׃