Yes, your gauge will be slightly different when knitting in the round versus flat.
Easiest ex: Most people tend to knit tighter than they purl. So, when working stockinette in the round one would knit every row. The swatch will be smaller than if one worked the stockinette flat and purled every other row which would make it looser/bigger.
The opposite is true for Garter stitch.
Any other pattern varies dependeing on the knitter and technique (Fair isle oftten comes out looser when worked flat and tighter when worked in the round, especially a small round).