Is it bad if a betas gills are turning red?

Yes! Very bad! It's a sign of ammonia or nitrite poisoning. Other signs are: gasping for breath at the water surface, being lethargic, torn and jagged fins, loss of appetite, often laying at the bottom of the tank, darker coloring, red streaking on the fins or body. Ammonia ad nitrite poisoning happens when the tank isn't properly cycled or if the betta is in a tank smaller than the minimum 5 gallons bettas need and the water isn't being changed often enough.
Ammonia build up occurs when the betta breathes, poops, urinates, and when rotting food is left in the tank. Test for ammonia and nitrite levels; they should be 0. Nitrate levels should be below be below 40.
If your tank is less than 5 gallons and doesn't have a filter, then do 100% water changes more often; don't worry about nitrite or nitrate. Consider getting a 5+ gallon for your betta because they can live up to 10 years if properly cared for.