What do you mean by thumb shifter? in bike terms, a thumb shifter pre-dates Gripshift, if you mean trigger shifters like Shimano Rapiidfire+, these are probably the most common type of MTB shifter out there.
Does the bike still fit you? it's 23 years since you got it, is it still comfortable? if not, don't put any money into it, if it is, would go with Thomas15's suggestions, + swap the tires for slicks or semi-slicks (dependant on the condition of that 3.5 mile gravel bit is like).
One issue you may run into is the fork/headset/stem, the bike is listed as having a 1 1/8" headset, this is an uncommon size (very short-lived as the industry (MTB) was almost all threadless headset/Aheadset by 1996) they do exist, but choice is severely limited, and if you need a shorter/different stem, which are even less common that headsets in this size, if you need to swap the stem. would look at an Ahead converter, these work fine (have put about 5000km on a bike with one fitted in the last 2 years)
If you do change the stem, would look for an Alu riser bar, as dropping the steel handlebar that is stock will drop a lot of weight of the bike. Fitting one with the stock stem may be problematic.