I own a GT mountain bike, I am not sure what year 1990? It heavy and right now it doesn't work. I need a tune up on the bike the gears will not shift. We have greased them a few times but they still fail. It has been tuned up in the past but that was probably over 10 years ago. I am a casual rider, just out for some fun off road speed it good but not the most important thing. I would like something lighter to.
So my question being is that should I spend $100 with parts & labor on my old bike that I am not attached to, or should I go out and get a new bike. My budget is no more that $200.00 and I don't want to go to Walmart or Target to buy a new bike, lol.
If you're not willing to spend more than $200, and don't want a department-store bike, then your options are severely limited. Either you'd have to buy used, which can be a goldmine if you know bikes, or an utter waste if you don't. Or you buy some new shifter cables, flush the shifters with wd-40, read and look at some net tutorials, and get the old beater going again. Maybe have a bike shop replace chain/cassette for you.
As dabac, dependant on the model, the GT could be worth reparing or not, however that value is only with the retro community, for the average user, looking at a more recent 2nd hand bike (going on the limited budget) would be a much better option if all you want is 'fun off road speed'
Fix it. Anything decent that is new will cost you $500. Anything other than a bike store sells crap. Used can be good but sounds like you would not know what is good. You describe yourself as a casual rider. If you off road ride then you need a descent bike. My brother bought a cheap bike and wrecked it on a casual mountain bike ride.