As far as in the gym, try the following exercises with light weights and see if they increase your ability to control your body and bike. These are the exercises you can do without machines, if you have a machine, you could hit the leg press machine which should help you on climbs.
Bent-over Rows for your back (helps hold you up and do aggressive climbs)
Squats for your legs (helps everything)
Lunges for your legs (helps your hamstrings and quads for upstroke pedaling)
Dead Lifts for your legs (helps your hamstrings and butt)