Merkava, it sounds like you haven't worked on toyotas.
The petcocks have spigots to which you attach a drain tube to, then you can put the hose to a container. It's not unlike bleeding brakes, where you can cut it off whenever you need to.
There is so much more to disagree on about the strategy other than the 1gallon measurement part; but if we're so set on debating this point:
a) use tube to put it into a container where he's drawn the line. turn off the petcock when he hits about 1gallon.
b) just buy an extra jug of coolant
c) add water if he's just a bit short on coolant, and end up with slightly diluted coolant.
d) dump back the portion of the old coolant he wants to keep so he has only swapped 1 gallon.
We have no idea what are the goals of the OP, it's recreational maintenance and use of his free time. The plan is already ridiculous, so additional ridiculous steps can be added to the procedure. But the requirement of "I want to swap exactly 1 gallon" can be fulfilled, and is not really a problem to raise an objection over