Why did he say "Al Tagidu v'Gas"?
Radak: He laments, [there was no need to tell there]; they knew, for they were killing Yisrael in the war. Also Michah (1:10) lamented "b'Gas Al Tagidu."
Malbim: Do not publicize that they fell due to Hashgachah, lest [Pelishtim] rejoice that Hashem abandoned His nation. It says Tagidu, for a Mevaser tells so people will openly rejoice and feast. A Magid did not see by himself, he only heard, or he does not intend to publicize. In Gas, the capitol city, a Magid told the matter, and [Pelishtim] sent to be Mevaser in all cities, that they should rejoice and feast.
Why does it say "Pen Tismachnah
Malbim: Alaz is one who dances amidst great joy. Simchah is in the heart. Do not be Magid, lest this cause Simchah, and do not be Mevaser, lest this cause Alizus.