Gain Attack is not applied to troops under Entangle status.
Easiest way how to test it is to get Dragotaur on 1st slot (or any other dragon on 1st slot, in addition to easily Mana-built Dragotaur) and play against Sylvanimora. Allow the enemy to get a 4/5-gem match and then boost the troop up. Wait till Entangle wears off. You'll notice there has been no effect.
Based on logic, Entangle is there to suppress the Attack of a troop. Therefore, increase of attack stat should work, even though the bonus obtained should stay suppressed for the duration of Entangle. Once Entangle Wears off, the bonus attack should be there.