You would take half with Gisela out, so just a single damage per nonbasic land you are running, while your foes double, so essentially four damage per nonbasic land they control
The same is true for counters that change the creature's power or toughness
If you talk to your playgroup to set some expectations first, they will hopefully understand why you want to start attacking early, turn every land into a Mountain, or block their extra card draw
Its hard to get too excited about Herald of Secret Streams when the majority of Merfolk are already of the evasion persuasion
The card notably doesn't affect you, so you're free to play all the even-CMC cards you want with it
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