Buying certain packs will unlock Gift Chests for your alliance. Each pack that will have a gift will show you the gift as part of the purchase. These chests are automatically "activated" and can be claimed under Alliance -> Chests -> Alliance Gift Tab within 24 hours. 

This gift chest can contain one of the following;
40-60 Gems (10%),
2,000-3,000 Hero XP (45%)
10-15m Construction/Training/Research Speedup (15% each)