If I play Grand Stratagem, then I play units after stratagem has resolved, do those new units get the +5 might?

No, the units played after Grand Stratagem has resolved do not receive the +5 Might bonus.

According to RiftJudge FAQs #8163, #8376, and #7861, Grand Stratagem creates a snapshot effect. When the spell resolves, it grants the +5 Might bonus only to units that are already present on the battlefield at that moment. Because the effect is not a continuous or passive aura, it does not apply to any units that enter the battlefield after the spell has finished resolving and been placed in the trash.

Sequence of events:

1. Grand Stratagem is played and enters the chain.
2. Grand Stratagem resolves.
3. The +5 Might bonus is applied to all friendly units currently on the board.
4. Grand Stratagem is moved to the trash.
5. If you play a new unit afterward, it enters the board after the effect has already concluded, and therefore it does not receive the bonus.