Whilst it isn't an excuse for the lack of this functionality on Android it should at least be possible to obtain it. Android gives you the flexibility to install a drawing app and for that app to appear when you choose to attach a picture to an email in the gmail app.
This isn't honestly something that interests me so I can't name a drawing app that does this. Still lots of file system apps (e.g. ES File Explorer) have handlers which appear on the attach picture screen, it shouldn't be pushing boundaries for an image drawing application to appear, let you draw, save the image, then pass it back to the gmail app.