I'll give it a try below. I hope by Jupiter that this is correct because I'm really dizzy, and these matters can be confusing as hell, even if you (think you) know them.
If you call more than 40%, then you'd be exploitable; the bluffer should never bluff (he loses $300 every time you call, but wins only $200 when you fold).
On the other hand, if you call less than 40%, then the bluffer could bluff 100% of his hands and show a profit.
So if we fix the calling frequency at 40%, what happens if the bluffer bluffs 100% of the time? He'll break even (0.6*$200 - 0.4*$300 = 0). Likewise, if he never bluffs, he trivially breaks even.
This independence of the result of his bluffing frequency shows that 40% is the GTO calling frequency. Note that the pot odds tells a different story, the "mnemonic" above (valid for bluff bet and call) would say call 60%. The difference is that it is much more expensive for the bluffer to bluff with a raise than a bet.
How often should the bluffer bluff? If he bluffs too often, you could exploit him by calling more, and if he bluffs too little by calling less. He is giving you 5:2 to call, so it should (as usual) be 5:2 against that he is bluffing. Let us fix the bluffing frequency at 2 in 7.
Now what happens if you call 100%? 2 times of 7 you will win 500 and 5 times of 7 you will EDIT:win -> lose 200. This is zero zip nada. The same as if you call 0%. Again, this independence of the outcome of the frequency (this time of the caller) shows that 2 in 7 (or 5 to 2 against) is GTO.
Hence, if you bet $100 into a $100 pot and are raised (all in) so that it costs you $200 to call, what are the GTO bluffing and calling frequencies?
The answer:
The bluffer should bluff 2 times for every 5 value hands, i.e. 28.6% (5 to 2 against or 2 in 7).
The caller should call 40% of the time (2 in 5).
Neither can deviate from these numbers without becoming exploitable. But both can deviate provided the other player does not.
These results are perhaps surprising, and the corresponding numbers for check-raise-bluffing are even more surprising. There it is easy to look too much at the pot odds for a call.