I don’t have the web links on hand, but others in the PC/Mobile threads have crunched some stats on obtaining Legendary or Mythic troops in general. You would be able to extrapolate the probabilities for pulling them from typical Glory chests.
Depending on the chest, I have heard odds in the range of 0.1% to 2% chance.
That being said, the probability of pulling Legendaries and Mythics from Guild Chests will be extremely low if you are talking about Level 3 chests (10,000 guild seals) that still contain the Guild Guardian commons.
The Guild Guardians (the white troops in Guild Seal chests) are coded to drop at a 50% probability until they become mythic for you. Then they will be removed from your drop pool.
As you can guess, your best chances of getting Legendaries/ Mythics will be greatly improved at that point. Meanwhile, there isn’t any real benefit to holding onto personal Seals until your guild finally achieves Level 6 chests weekly (40,000 seals). Even then everyone will still need to keep drawing the white Guardians at a 50% drop rate until they are mythic.