I look a look at the base stats of the Tyranitar and Gigalith (megas not a factor) and actually was surprised, Gigalith has higher attack (by 1 point, small but still higher) significantly higher defense, but it has slightly less HP, think of it like a Rock type metagross.
This means that if Gigalith gets smack down and at minimum, gets stone edge, it actually will outclass tyranitar in DPS as a rock type, possibly outclass it at TDO, and possibly in CP (gigalith's stats are very similar to, and even slightly higher than, metagross) this isnt to say double rock Tyranitar is useless, it is still very strong, it is just that there could be something better out there when gen V rolls around, that is also easier to get access to.
Actually Gigalith's abysmal speed (base 25) lets it down here, since I just calc'd its base stats being curious myself and got
229 Atk, 222 Def + 170 HP
vs Ttars 251 Atk, 212 Def + 200 HP
So in order for Ttar to lose the TDO advantage it would need to go up against (Flying/Ice/Fire)+Fairy type, a bug type or a Bug+Fairy type
Atm there are 3 fully evolved Pkmn that match these dual typings, being Alolan-Ninetales, Togekiss and Ribombee (and ofc the bug types)
You really should understand the base stat formula conversion for POGO before you try to predict future gen pokemon's impact from their main series stats.
Just look at the POGO stats since they have all been calculated already. Whichever pokemon it is, you can google "[pokemon name] pokemon go" and you will get a result on the first page that has their POGO stats. Gamepress already has the base stats for all gen 4 pokemon and there's an article on pokebattler that lists all the highest attack stat mons per type per generation up to gen 7. Basically everything with the slightest bit of potential has been mentioned in that article, it's an educating read.
