The gearbox will have to pretty much come apart completely to replace reverse, so it'd be a good time to renew bearings etc. If the reverse idler gear (the one on the short, pressed-in shaft inside the box) is damaged, be sure to check the condition of the corresponding gears on the layshaft and mainshaft.

With bearings, most of the needles and ball bearings should be available at reputable bearing suppliers, at a much lower cost than genuine items. One exception: don't be tempted to get a "cheapie" bearing for the large one on the input shaft. Less-than-original quality ones use a sub-standard quality plastic for the ball cage, which is liable to rapid deterioration. You'll have to bite the bullet and pay for a decent one there. The two thrust washers on the layshaft might have to be OE too, unless you can match them up to a GM version (I did here in Australia, but that might have been a local thing).

There's only a couple of gaskets, so I usually make them from oilproof paper of the correct thickness. The top cover gasket often survives intact, so can probably be re-used.

If you have a good service manual, gerabox rebuilds are pretty straightforward provided you take your time and follow the instructions implicitly.
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Steve.