I think it's about time the df and du commands (executables?) were updated to display gigabyte units without one having to do their own calculations from the -m option (display in megabytes). This is one of those "little nothing" annoyances that, at the same time, would positively show new Linux/Unix users that, from pillar to post, stem to stern, sub-basement to penthouse and at all points in-between, open-source solutions keep up to date with the times.
Yes, and there's a reason they put it in a switch. Most commands available in shell are designed to output data extremely consistently so other programs can use it predictably. If you know it'll always report 1K blocks, that's easy to build a shell script around -- but trying to parse mixed units like 115K, 2.2G and 441M would be a nightmare.
Lastly, always, always, always read the man pages before complaining about a lack of feature. You'd have found the -h flags in both commands. You'd also have found email addresses to report bugs to.