Not at all — post-mount hydraulic brakes deliver the same stopping power and modulation as flat-mount. The difference is really just the caliper attachment style and a slightly bulkier look at the brake. You won't feel any performance gap on the trail.
Where post-mount actually shines is compatibility. Because it's been the mountain bike standard for years, replacement calipers and parts are widely available, often at lower prices than flat-mount equivalents. If you ride a gravel bike with MTB-influenced geometry or an older frame design, post-mount is likely what you'll find, and that's perfectly fine.
The only real consideration is frame compatibility — you can't swap between mount types without an adapter or a new frame. So if you're building up a bike from scratch or planning future upgrades, just make sure your caliper type matches your frame mounts. Either way, you're getting the hydraulic performance that matters for gravel riding.