I knew this might be a bit difficult for some people to comprehend that is why I had to remove some complex processes involved. But will try and explain it better here, thanks for the question

The gametocyte are like married couple male gametocyte (husband) while female gametocyte (wife) that need to have sex to produce children, and the only suitable place they need to have this sex is in their room on bed. Now for these male and female gametocyte they need to have their sex in the body of a mosquito, this is the only suitable place for them. This simply means they must be transported into the mosquito and this process occurs when the mosquito is sucking blood from humans being, since they in our blood and the mosquito is sucking our, they are sucked along in the process. If they are not suck from our body into the mosquito, they will die off after some days. So like I explained above, drugs that kill gametocytes can be used to kill them in the body.
Hope this explanation is okay.... If you still have questions, pls do ask.

To answer your second question, usually some people can be bitten by mosquito and yet not go down with malaria sickness. This is is because they have strong immune system (the ability of the body to fight against infection) . Now when your immune system is down you are at a high risk of going down with sickness or malaria when you are bitten. You might be having the signal or symptom and decide not to do anything, well the body can take care of the parasite and kill it simply because the body immune system is strong, but when it is weak the hypnoizoites in the liver which have been blocked by the body's immune system will be reactivated and then strike. This time around, they will strike with full force and if proper treatment is not taken, one can be critically ill. At this juncture I would say yes, stress can contribute to the illness because during stress your immune system is down. One need enough rest and Fruit.
Hope this also answers your questions.
Thanks for reading @arinzechukwu