Tire clearance for the frame will be the biggest hindrance to an older conversion. You will need to figure out what the max tire size the frame can accept, and most road bikes from 10+ years ago were in the 19-23 range, which really arent that wide. You will want something with 28mm at least, and even most road bikes with rim brakes from 5 years ago are barely accepting these. Wider tires means lower pressure, which is what you want for gravel. Most gravel bikes today are accepting 30-45mm tires with disc brakes.

As the earlier poster mentioned, tire clearance is going to be your big issue. I'm not sure what kind of gravel you are riding, but you should probably go with at least 35mm tires unless it's just hard packed dirt all the time. If you get into anything loose, muddy, etc. you're really going to struggle with 23's or 25's. I picked up an old CX bike for around $700, complete with a second set of wheels so I can have one for road and one for gravel. Sure, it's not as awesome as some of the newer gravel specific bikes like the Salsa Warbird, but I'm good with rim brakes in most conditions and can fit up to 40mm tires on it.

Tire size and gearing will depend on terrain and type of gravel. If you are looking at packed dirt or smooth gravel you will probably be ok with 28 mm tires, but rougher than that or sandy/muddy you will do better with bigger tires and some tread. For your gearing you probably have some flexibility with your current equipment. If you have a 110 BCD crank I would try to do 50/34 rings with 11-32 cassette. If you need lower than that you will probably need to look into a new crank or derailleur.

I have ridden plenty of gravel on a roadie setup.
Get the widest tires possible. 25C is probably as good as it gets.
Maybe throw on some wider bars for a bit more control.

Pick your lines carefully and except to get a little beat up.
There is some rocky gravel around here with lots of baby heads... most of my buddies on road bikes punctured there.

Putting on the biggest tires you can manage, and the lowest gears. I think bigger tires with minimal tread is better than narrower tires with large tread.

I've got a steel road bike with 28s up front and 32s in back that I use on some non-rocky fire roads and trails. Very fun.

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I recently saw a post on a gravel cycling site (unfortunately can't find it now) that cautioned against really tight clearances with carbon because the mud, sand and rock debris sticking to your tires can wear at your fork & seat stays like sandpaper. I don't have any first hand experience but seems relevant so thought I'd throw it out there.