Hi, i have a marshall 1959slp reissue, which does not offer as much gain as i sometimes require. I have 200USD and i am looking for a good gain booster to work with this amp to get sound like gnr, randy rhoads and dream theater.
Pardon my english.

Epiphone les paul standard plustop pro (LH)
Marshall Vintage 1959slp reissueBoss DD-3
Jim Dunlop GCB95 wah pedal
You can never go wrong with a Way Huge Green Rhino.
DT sound might be a bit tricky to pull off either way though with that amp, but the Green Rhino could sure help.
The mid curve and the 100hz cut/boost is incredibly helpful as well if you want to sculpt your sound before it hits the amp.
Things with strings:
Ibanez J.Custom, Prestiges, RG8, SR5 bass etc
LP's, Strat, Tele
ENGL Retro Tube 50
5150 III 50W
Orange Terror Bass
I recommend a zvex sho! It's one of the best and loudest boost pedals, and also really transparent.
Hardwire CM-2
Ibanez TS9
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
gain booster is a very very general description. an overdrive? distortion, clean boost?

it depends how you run your amp. if you mean a boost or clean boost, are you boosting a distorted or crunch channel?
Carvin CT624
Walden G630ce Acoustic
Carvin V3M, Avatar 2x12 WGS Reaper, vet 30
(crybaby, Fairfield circuitry Comp, GFS tuner, Vick Audio 73 Ram's Head, Xotic AC booster, lovepedal trem, TC Flashback, PGS Trinity Reverb, Walrus Audio Aetos power)