You're right, that word is used in 2 Peter 2:4. However, read the context of what it says.:

For if God did not spare angels when they sinned, but sent them to hell,[a] putting them in chains of darkness[b] to be held for judgment; 5 if he did not spare the ancient world when he brought the flood on its ungodly people, but protected Noah, a preacher of righteousness, and seven others; 6 if he condemned the cities of Sodom and Gomorrah by burning them to ashes, and made them an example of what is going to happen to the ungodly; 7 and if he rescued Lot, a righteous man, who was distressed by the depraved conduct of the lawless 8 (for that righteous man, living among them day after day, was tormented in his righteous soul by the lawless deeds he saw and heard)— 9 if this is so, then the Lord knows how to rescue the godly from trials and to hold the unrighteous for punishment on the day of judgment. 10 This is especially true of those who follow the corrupt desire of the flesh[c] and despise authority.


The pattern of what the text is saying is:
1. illustration of one group of beings that transgressed
2. illustration of the punishment they received
3. illustration of another group of beings that transgressed
4. illustration of the punishment they received

In each case, both the beings being referenced and the punishment are difference.

We see fallen angels being locked up in tartarus. We see ancient peoples being punished with a flood. We see Sodom and Gomorrah burned down. Each time it is an example of one group of beings sinning and being punished for it, each time differently.

Had the translators of our bible been consistent with their choice of which word to translate hell and had either left tartarus as "hell" and gehenna as "The Valley of Hinnom", or had left gehenna as hell and tartarus as "tartarus", would you have made the connection that these are the same place?

Even sticking to a literal interpretation rather than taking into account the hellenic culture 2 peter was written to, it would be difficult to conclude that tartarus and hell are the same place. One is a place that as far as we know only angels have ever been sent to. The other is the city dump outside the walls of Jerusalem.