During my 1st main phase I cast Walking Ballista for X=20 (cmc=40). Walking Ballista on the stack, no response. I get priority back. I announce that I'm holding priority and remove each of the 20 counters targeting my opponent. Will he have the gap to cast krosan grip and stop it?
No, your opponent is out of luck if you do it correctly. Because after activating an ability, the player who did so regains priority. So you can put a whole bunch of Ballista activations on the stack before your opponent gets a chance to do anything. BUT, you have to respond to your own action RIGHT AWAY, you CANNOT wait to see if there are any responses! None of those activations will resolve yet, but since they are independent of their source, destroying the Ballista in response won't stop them from killing your opponent. He has to deal with those activations directly (a whole bunch of Stifles or a Time Stop), or with you (if you lose, you take everythig you control on the stack with you).
If you cast Ballista, then ask for a response, when your opponent says no response you don't get priority back while Ballista is still on the stack - instead, the Ballista resolves and enters the battlefield, and only then you get priority back (assuming it is your turn).
But yeah, after the Ballista has entered, you gain priority and can activate its ability 20 times, announcing you're holding priority after each activation to activate the ability again. Your opponent will only get priority back to respond when you're done activating 20 times, and at this point the Ballista is on the graveyard already because it reached 0 Toughness (and even if you had activated it only 19 times or less, destroying the Ballista won't counter the abilities already on the stack, so Krosan Grip can't save your opponent either way.)
During my 1st main phase I cast Walking Ballista for X=20 (cmc=40). Walking Ballista on the stack, no response. I get priority back. I announce that I'm holding priority and remove each of the 20 counters targeting my opponent. Will he have the gap to cast krosan grip and stop me?
No, he won't.
Detail in case you need this in an argument, using X=2 to shorten the example:
Your main phase begins.
You get priority and cast Walking Ballista. You choose X=2, and you pay the cost using 4 obtained in an unspecified way.
You get priority and pass.
Opponent gets priority. Opponent might like to cast Krosan Grip, but the Walking Ballista on the stack is not a valid target (yet). Opponent passes.
The top object on the stack (the spell cast in step 2) resolves. You put Walking Ballista on the battlefield with 2 +1/+1 counters on it (making it a 2/2 creature).
You get priority and activate the "Remove a +1/+1 counter from Walking Ballista: It deals 1 damage to target creature or player." ability of Walking Ballista. You choose opponent as the target and pay the cost by removing a +1/+1 counter (leaving it a 1/1 creature).
You get priority (relying on your announcement of holding priority to avoid a standard shortcut) and activate the "Remove a +1/+1 counter from Walking Ballista: It deals 1 damage to target creature or player." ability of Walking Ballista. You choose opponent as the target and pay the cost by removing a +1/+1 counter (leaving it a 0/0 creature).
You would get priority, but there is a state-based action to perform. Walking Ballista is a creature with a toughness of 0, so it is put in your graveyard.
You get priority and pass.
Opponent gets priority. Opponent might like to cast Krosan Grip, but the Walking Ballista in your graveyard is not a valid target (anymore). Opponent passes.
The top object on the stack (the ability activated in step 7) resolves. Walking Ballista (as it was in step 8) deals 1 damage to opponent.
You get priority and pass.
Opponent gets priority and passes.
The top object on the stack (the ability activated in step 6) resolves. Walking Ballista (as it was in step 8) deals 1 damage to opponent.
You get priority and .... (Unless this has been enough to reduce the opponent to 0 life.)
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But yeah, after the Ballista has entered, you gain priority and can activate its ability 20 times, announcing you're holding priority after each activation to activate the ability again. Your opponent will only get priority back to respond when you're done activating 20 times, and at this point the Ballista is on the graveyard already because it reached 0 Toughness (and even if you had activated it only 19 times or less, destroying the Ballista won't counter the abilities already on the stack, so Krosan Grip can't save your opponent either way.)
No, he won't.
Detail in case you need this in an argument, using X=2 to shorten the example: