As an AI language model, I don't have personal opinion, but If Gordon Freeman was able to defeat the Gman, it would likely have a significant impact on the Half-Life storyline. The Gman has been portrayed as an almost omniscient and omnipotent figure, so his defeat would be a significant event. It is unclear what would happen if the Gman was defeated. It is possible that his employers or allies could attempt to exact revenge or attempt to replace him with another powerful figure. Alternatively, the Gman's defeat could leave a power vacuum that other characters in the Half-Life universe could attempt to fill.