I'm mulling over the idea of trading my backup/rain bike for a cyclocross bike or just buying the max size xc tire it can handle. At the end of the month I am closing on a house quite close to the beginning of a 60+ mile rail trail with good terrain and small gravel. The bike is a 2008 Giant TCR Advanced 0, Rival equipped. It won't take much abuse on this trail and I lean toward the tire option if at all possible because I don't want to get rid of the bike. I haven't researched the width it can handle nor do I have any experience riding a cyclocross bike for comparison. Thanks for your input.

Personally I'd try the tire option first, see how that goes and then evaluate.

If I had the clearance on either of my road bikes, I'd be running 700x45 Compass Baby Shoes but sadly 700x28 is about as big as I can fit.

Semper Fi

brakes will be your issue.  If you already have disc or cantilever, you should be fine.

my cyclocross/gravel bike is just a road bike with disc brakes and knobbies.

I'm reading about some clearance issues with certain 28 tires but none with other brands. Some Rival standard brakes are good up to 32mm tires . I don't remember if the calipers are short reach or standard so I'll have to check brakes and fork when I get home and see what the limits are. Is a 28mm xc tire going to handle small gravel well enough?

If the frame will handle bigger tires than the brakes will allow, you'll just have to pass the tires thru the calipers deflated, then reinflate after you mount the wheels.

The fork looks like it would go 30 or 32mm but the brakes don't look like they would allow anything larger than 28mm. The extra 5mmthan I'm used to just doesn't seem like it would make all that much difference in gravel but I wouldn't really know until trying. Still better than trying a whole new bike first.