Everyday you can get 30 gems by doing 7 daily quests. Also now and then you can collect gems on the Daily Rewards page. In 28 days, you can get 1000 gems through Daily Rewards, and 840 gems through daily quests.
When you start, it’s best to keep hold onto your gems until you are level 65. At level 65, you can buy Emblems with gems. 1000 gems will give you 5000 emblems. With those 5000 emblems, you can make an exchange at the Exotic Merchant to receive 3 epic chests. Don’t spend more than 2000 gems on emblems.
At level 120, Dragon Blood will unlock in the store. From this point on it’s better to spend your gems on Dragon Blood. You can only get 200 Dragon Blood per day through missions. With 1000 gems you receive 3600 Dragon Blood. That’s 18 days worth of Dragon Blood.