No, definitely no issue with gas exchange. I've run a 24g nanocube with too many fishes, and currently have a 29g biocube.
You'll get plenty of gas exchange as long as you have some flow in the tank (stock pump is plenty). If you really get crazy into the tank, you may end up doing a skimmer in there, and then you definitely will have nothing to worry about.
But stock setup, I've never heard of anyone having any issues.