...Forza gets talked about a lot because it's the name of two series of games and they release one every year...

That is essentially half of my point.  Forza is the big name in console racing sims right now.  Certainly, GT will outsell Forza, since the install base of the PS4 is much larger, and the games only come to market once every few years.  But, it has lost mind share to Forza in recent years.  So, I don't think it will sell like it did a decade ago.  

That is essentially half of my point.  Forza is the big name in console racing sims right now.  Certainly, GT will outsell Forza, since the install base of the PS4 is much larger, and the games only come to market once every few years.  But, it has lost mind share to Forza in recent years.  So, I don't think it will sell like it did a decade ago.  

That is essentially half of my point.  Forza is the big name in console racing sims right now.  Certainly, GT will outsell Forza, since the install base of the PS4 is much larger, and the games only come to market once every few years.  But, it has lost mind share to Forza in recent years.  So, I don't think it will sell like it did a decade ago.  

and why did gt 6 sold more than any fm on x360? userbase?

But seeing FM 5 and 6 selling about 2M on the X1 userbase that now is around 30M how much does X1 have to sell for Forza to sell good? Why the other 28M that didn't buy it aren't enough to propel the sales?

I'm not really sure what you're talking about.  

I have not said anything about Forza selling better than GT, even relative to install base.  I've simply said that Forza is the game that we hear the most about over the last several years.  GT was a juggernaut in years past.  It may still be.  But, it is a fact that Forza has gained a lot of mindshare in recent years.  That is a negative for GT sales, to some degree.    

Hard to say but I'll go with 6 million~ because genre decline + poor marketing + limited content + different approach.

With that being said, Gran Turismo is a much bigger IP than Forza. And that has little to do with insall bases. A lot of gamers over emphasize on attach ratios and choose to ignore more important details.

I probably said this before but I'll say it again:

"Persona didn't become 15x bigger when P4G had 15x better attach ratio than original P4 despite being a mere enhanced port."

Gran Turismo's fanbase itself, dwarfs that of Forza. This comparison takes place in a space unrestricted by hardware install bases. Dragon Quest XI's PS4 numbers were surprisingly close to the 3DS version even though it's more expensive, on a much smaller install base, and can't be played outside your room.

The thing is.. popular franchises with passionate fanbases follow their own set of rules and have a tendency to hit a relatively fixed number, per mainline iteration, as long as quality remains somewhat consistent. They function almost independently from install bases, and are more affected by whether there's another mainline iteration in the same platform. MGS's sales were extremely consistent over the generations because it's for the most part the same playerbase carrying over to future systems. PS3's tiny little install base at the time didn't stop MGS4 from selling. And no, that didn't mean the series became twice as popular than it was in the PS2 era either. There is a lot more you'll need to take into consideration than combined sales and attach ratios to gauge popularity.

I'll say Gran Tursimo's popularity is probably in a significant decline, but it's nowhere near enough to put it close to Forza, let alone a declining Forza! Only when a proper Gran Turismo 7 fails to sell 3-4 million, will I acknowledge that Forza is more popular lol.

You are honestly expecting it to sell less than a Prologue did on PS3 or a spin off on the PSP?

GT6 which people even here didn't even know exists that launched as a PS3 exclusive against the PS4 massive launch and shift of interest focus has crossed 5M. GTS will sell well over 3M in Europe alone. Think, man!