i own a Behringer GMX 212, and YES, I am well aware of Behringers reputation as a less than mediocre Guitar and amp brand.
I've had it for about 3 years, and apart from the gain not being as high as I'd like, I'm quite satisfied with it.
My problem is; When I crank it past about 5, it won't go any louder, even with the preamp on my Boss ME-70 multi FX pedal. I've gigged it a few times, and it always gets drowned out by my mates Peavey Viper.
I want to know if there is any way to customize it to make it louder, with more gain, or if I should just replace it.
This is the wrong section you need to be in GG&A.

But to answer your question, get a new amp. If you care to start a new what amp thread UGer's can help but put it in GG&A
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
replace it. the only real way to make an amp any louder is to replace the speakers with more efficient ones, but that will probably cost you at least the value of the amp.

that being said, that amp should be loud enough (lots of guys gig with 20-30 watt amps with no trouble at all). try playing with your eq; what sounds good at bedroom levels playing by yourself will be too bassy and muddy to play with a band at gig volumes (equal loudness contours at work here). try adding mids and some treble, it'll help you cut through the mix.