I heard some rumors about Lexus is planning for the next-generation GS (the BMW 5-series' rival)



Here are the words what would the next-generation GS would told:

(Source: Motor Trend)

Gone is the current 4.6-liter V-8, replaced by a 2.5-liter V-6—most likely a version of the engine presently found in the IS. The present 3.5-liter V-6 will become the top of the range. About a year after launch, a 2.5-liter hybrid will reportedly be added to the GS lineup.

In contrast, the GS F variant will employ the IS F’s 5.0-liter V-8, but with strategic modifications including revamped ECU and exhaust. Horsepower will reportedly be boosted from 417 hp to as much as 464 hp.

...the GS F will also incorporate a lighter platform and some carbon fiber body parts to get pounds down to a target weight of around 3530 lbs.
That's not all, they're planning for the GS-F, which the engine from the LFA would be used for this: