Don't gem buildings,
Get the rest of your builders first thing.
From there you should only gem lab time (when you have enough for the next research) and boosted barracks.
When you boost barracks be sure to have a builder free for walls as you will most likely pull 4-6 million of each resource (after which is also a good time to gem research)
Research is by far the biggest bottleneck at 8. You'll burn through the gems way too quickly if you do buildings (to start at least) and be stuck with a ton of research left

