The verdict
If you often need a quick pitch outline, internal update or one-pager, and momentum matters more than pixel control, pick Gamma.
If your decks leave the company as files, need a strict brand template, or must work offline, or if you already pay for Microsoft 365, pick PowerPoint.
You can combine them. Draft quickly in Gamma, export, and polish in PowerPoint. That works well for a one-off pitch. For decks that customers or a board will receive, I'd keep PowerPoint as the default, since the file format is the standard. Use Gamma to get past the blank first slide.