If the guitar instructions said use lights, under NO circumstances use mediums at standard pitch. You will belly it, fold it, or rip the brigde off.

There are two standad scale lengths for US built middle-aged guitars, 24 7/8ths inches and 25 1/2 inches. The difference is not enough to let you get away with mediums on a guitar that is specified to need lights.

Get the action right. If the action is too high and there is not enough spare tomake the adjustment at the saddle, you will need a neckset. Check this first.

Then, first step, neck relief. Put a capo on so the strings are fretted at the first fret. Use your finger to fret the bass string at the point the fingerboard goes over the body. At the halfway point between the two you want just enough clearance of the string from the frets so that it is almost easier to tell by tapping than by looking by eye to check whether the string does in fact clear every fret. Ie the figerboard has a very slight forward curve while the neck is under string tension. If you play VERY VERY hard you may want a little more curve but only a smidgeon. In theory the strings should leave the fret at the same angle all the way along the neck.   If you have a truss rod, adjust this curvature with it. You may need to slacken the strings, tighten the truss rod,then tighten the strings again, possibly repeatedly. I think all Gibson trussrods are right hand thread.   Some of them have a trussrod nut at the headstock end that is a little sod to get a socket spanner onto properly, and you may need to buy the special tool for the job, usually available from Stewart-MacDonald.

If you have a non-adjustable neck and the curvature is more than that, it is possible sometimes to re-fret with frets with thicker tangs to straighten the neck a bit more. It is also sometimes possible to plane the finger board to correct profile, or to cap it with a tapered piece of wood to avoid having to do a neck set. These options require great expertise and a fair expense.

Once the neck profile is right, do the saddle height. The profile of the saaddle needs to be determined by teh radius of the fingerboard, so that there is a nice steady progression in action at the 12th fret, probably from about 1.6 mm on the treble side to about 3 mm on the bass side - less if you play very lightly and want the action to be easy (BUT THEN THE GUITAR WILL BE QUIETER), more if you hammer it like hell or play bluegrass bass runs. You may want to put B-compensation in the saddle at this time. You should now have a guitar that, if you capo it at the first fret, plays evenly all the way up with no rattles until you get carried away.

Now and only now do the nut. You will ideally need nut files but you can manage with a selection of files and small hacksaw blades although the rounded nut slot you get with the right tool is a plus.

There is only one "right" height for the nut slots. It is so that the action at the first fret with no capo on is the same as the action at the second fret with the capo on the first fret. However, if you habitually dig in much harder on root chords than up the neck you may want it about half a thou higher.

If this does not result in the guitar playing right, you have either a twisted neck or frets that need stoning or replacing and then stoning.

Until the guitar is playing right, you cannot really judge the sound, or try to select strings that suit the guitar. Having said that you may want to try a lighttop/medium bottom set (most of the usual makers do them). I find the Elixir Nanoweb 12/56 both smooth and present in sound, and they last well too (even me belting them with a flatpick) - but if you use metal fingerpicks you may fray the coatings too fast. If so, try D'Aquisto Brass Masters - for me they twang a bit less than the Phosphor-Bronze. Others like the NewTones.

Do make sure the inner bridge plate is in good condition. If the ball ends have, over time, ripped it up you can try EZ-Pegs to get the balls to bear the other side of the plate, or there is a cunning gadget - a thin brass plate - that you can get on the internet. It is put with suitable adhesive (and filler if the bridge plate has really been chewed up) on the inner bridge plate and it is shaped to fit a bit round the pins. Thus the ball ends bear on the brass plate, so there is no cushioning caused by chewed up bits of wood.

Also make sure your saddle is tight-ish in the bridge plate. You should not be able to pull it out with fingers alone. Some say you sould, using pliers, be able to pick the guitar up with it but I think that it soo tight. Do make sure there is no fluff or grit or other stuff under it - you need a good continuous contact from one side to the other. I theorise that a three-piece saddle ought to create even contact between saddle and bridgeplate for all strings, but some think this heretical.

Make sure, if the guitar has been polished with wax polish, that you have all wax removed from the top (face). It makes a non-resonant coating that deadens the sound a bit. The best tool is a luthier's buffing wheel, but massive patience, elbow grease, and soft lavatory paper will get there in the end...

If this still leaves the guitar too quiet, or with insuffient bass for you, it is time to get a new guitar.