What do you mean by when? I'll try and clear things up for you.
In the original timeline the only gates made are the black gates that take the Gurus and Janus away, and the gate that sends Kino to 65,000,000 B.C. As for the creator of the gates it could have been Lavos or the Entity, it doesn't matter which. So anyways Lavos goes ape and busts up the planet. When the planet is on its last legs it trys to fix the problem that is Lavos.
It frantically searches throught its entire history looking for anything that can be helpful.
1000 AD: Looky here, this teenager Crono, he definietely has the potential, and the ability to use magic. Oh look! That girl he is with also has the magic ability. What the hell? His other friend has the ability too! This is too good to be true.
600 AD: Hmm that Frog character is one of the best swordsman on the planet. And he also has the ability! He also is able to wield that legendary sword that is tied into Lavos... What? That boy that dissapeared from 12,000 ended up here? Wow he has awesome magical power, and has a personal vendetta against Lavos. This is getting easier by the second.
2300 AD: Hmm this robot has a very unusual human like aura. Almost as if it can feel emotions. This is right about when the human race dies off also, and that old man was sent here through the gate. A very important time indeed.
65,000,000 BC: Amazing, that woman Ayla has awesome strength, super human almost. This period is when Lavos landed also, maybe it is at its weakest at this time?
The Entity then concocted its scheme to get the seven chosen together to fight Lavos. As soon as it had its plan set it struck into action. It went back to the point where marle entered the telepod and made a gate, covering it by making the witnesses think that the telepod and Marle's Pendant caused the incident.
You have to see the entity as having the ability to watch all of time from one place. Like a computer with all the files being the events that transpired on the planet. The Entity can not do much physically, but it can make gates, and has some control over the DBT. Hope thaty cleared things up.