Do not introduce `k` and `x` with floats. Use exact quantities instead. That should get around the discrepancy in Maple 9.5.

restart;
k:=99/100:
m:=1:
x:=(Pi*csc(Pi*(k-m)))/(693/1000*GAMMA(k)*GAMMA(m)):
m1:=MeijerG([[-m],[1-m]],[[0,-m,-m],[k-m]],(-m*k)/snr):
m2:=MeijerG([[-k],[1-k]],[[0,-k,-k],[m-k]],(-m*k)/snr):
c:=x*((((m*k)/snr)^m)*m1-(((m*k)/snr)^k)*m2):
plot([Re(c),Im(c)], snr=-5..10, color=[red,blue]);

In particular, when using those floats in the definitions of `k` and `x` as you had it originally, Maple 9.5 will produce this,

restart;
k:=.99:
m:=1:
x:=(Pi*csc(Pi*(k-m)))/(.693*GAMMA(k)*GAMMA(m)):
m2:=MeijerG([[-k],[1-k]],[[0,-k,-k],[m-k]],(-m*k)/snr):
evalf(eval(m2,snr=5));
259.8556695 + 5.313756679 I
exactm2:=convert(m2,rational,exact):
evalf(eval(exactm2,snr=5));
6.225950422 - 2.656878345 I

The above may not just be a roundoff issue -- ie. increasing precision might not fix it. You might have to use exact rationals in the arguments to MeijerG.

An alternative workaround for Maple 9.5 could be to convert your `c` (or perhaps just `m2`) expression to exact rational prior to plotting, manipulating expression `c`, or querying `c` at float values of `snr`.

acer