You can use any gem level on the ancient item, it's gem level *5 + gem type gives you the stat bonus on the ancient item.
For example, if you use (any gem) level 70, for example + 3 Emeralds + ancient item to be enchanted. Your item is transmuted to have +350 dex.
You can re-enchant over the item using the same recipe again.
The lev requirements on the wiki is minimum gem level needed to enchant item.
This post was edited by dcbc on Feb 18 2016 01:22pm