I see lots of questions on gears slipping but not sure it's the same as what I have. Out of 21 gears I only have problems with 4 (middle sprocket (2) on left shifter gears (4-7) on right shifter). Since it's only 4 gears I didn't feel comfortable adjusting anything since it may impact what's working now. I'm not sure what to look for but the sprocket, gear and chain wear look okay. I just bought the bike a couple weeks ago (used) and it shifted good through all gears. Bike is 2011 GT Aggressor that looks like new so it's not one of the 'mart' bikes. Any ideas? Thanks in advance.
This sounds of cable stretch. For new bikes I like to completely retune the rear dérailleur after a couple of weeks of riding. For used bikes having this problem I also check that the hanger is straight. Checking the hanger requires a special tool, so a trip to your LBS is probably in order if tightening up the cable doesn't do it.
Some folks will tell you to just turn the barrel adjuster for a stretched cable...my $0.02 is that the barrel adjuster is to allow you to make adjustments if needed during a ride. If you "use up" all the turns the adjuster has before your ride...well.
Adjusting for tension is pretty easy. Shift all the way to the smallest/hardest gear. Look at how the cable is routed...you aren't going to take it off, but just in case it gets loose, you want to route it back the proper way. Now loosen the hex bolt holding the cable. It should be a 5mm hex, just a bit should do it. Pull the cable tight. You can use a pair of needle nose pliers to help, I prefer to use fingers. If I'm not getting it tight enough, you can push the derailleur in a gear (or two depending). Tighten bolt onto cable. If you pushed on the derailleur for extra tension, pedal a bit to make sure it drops back to smallest cog (if not, loosen cable and try again...this time pushing derailleur less or not at all). Make sure it shifts all the way through...you shouldn't have to change stops, but check them. Shift to center of cog and adjust (you can use barrel adjuster here) chain so it's centered on cog (moving chain is easiest to see). Shift up and down several times, everything should be smooth.
If the cable tension doesn't work...it may be a bent hanger...stop by your LBS. The other possibilities are worn chain and/or worn cassette. Check the chain with a gauge. Cassette your LBS will be able to tell you...I'm guessing it's not this because it shifted good when you first got it.