If it wasn't so sad I'd lol hysterically, whenever someone comes out to say that you need to go online racing.
GT5 is a game that
CAN be played online, not
must be played online, so basically, while online racing is a nice-to-have, the games quality is still to be judged with the LAN cable unplugged and that's when the excrements collide with the rotating cooling device.
Let's have a look at the game if you play it offline, shall we? Let's see if grinding is necessary:
Pretending you are REALLY good and golded all licenses and all special events available thus far, you'll be somewhere near LVL 20 with a Million in your coffers and a dozen of cars in your garage.
That's where the first problem hits. If you already have installed the "evil patches" (1.06 and 1.07), you will have either no chance to gold some licenses and events or you'll have to repeat them endlessly until you somehow scrape below the gold time by sheer luck - I consider that forced grinding. This is, because they now FORCE YOU to use the ridiculous ASM electronic nanny in licenses and special events, which makes some events ridiculously difficult. Sorry Kaz, if I want to be constantly told what I can do or can't do and want to be subject to arbitrary limitations, I either buy an Apple or relocate to Belarus. 👎
And before the first one comes out of the wood work, telling me that I'm not forced to gold everything and that I'm unworthy anyways and gold is only for the "real l33t chauffeurs": A game that opens its full content only to a small minority of its users can safely be called mis-designed. I'm not saying you shouldn't have to work for the golds, but they should be realistically attainable, which some of them are not.
Back to the topic at hand. So let's say you've golded everything that has been unlocked so far, then you'll faceplant the brick wall very soon again, when trying to run all the races in the Beginner hall. You'll be missing cars for the 2 historic race events (european, japanese). Now you have two options:
a) you keep advancing days until some appear in the UCD. That in itself is a grind - a repetitive, forced upon you, time wasting activity.
b) you delve into the ridiculous B-Spec mode until you have advanced your driver enough to win the Dome Zero. That's all jolly funny, if you find it acceptable to buy a "real driving simulator", only to sit and watch an incompetent binary dimwit hurling its car into the armco for hours. I'm not ok with that and watching B-Spec mode is just another form of grind.
If you make it past this point without losing your last bit of faith in the fanchise, you'll hit another brick wall (i think) after the Indianapolis endurance race, when you only have some endurance races left and your level does not suffice to enter them. That's when "the grind" begins, because you'll have to re-run races just to advance your XP-meter. And it's rightfully called a grind.
Now some might argue that you reached that point in every GT title, which is only partially true. I never had to grind races just to be able to enter the next one. The licenses took care of that and besides, unless you restricted your GT5 playtime very heavily, not more than a month or two will have passed since your first A-Spec race. That's the end of the offline career. I seem to remember that GT2 lasted me several months and I was the worlds most laziest student back then, so I had VERY MUCH time to play.
So here we are at the long winded stories conclusion: In offline mode you'll have to grind heavily in many different facets if you want to enjoy all of the games meager content. The online racing might save it for those, who have a broadband internet connection (which not all have), but as an offline product GT5 is broken, flawed and not worth even half the money we've payed for it.
Take a look at Forza 3's event list - That'll keep you busy for many months, so GT5's shortcomings are not down to technical limitations, they are a result of uninspired and inept design.