I may be mistaken, but it looks like you want to compute quantiles at regular intervals ???
If it is the case, the following may help you
(after loadings of the suitable packages)
q := i -> Quantile(GammaDistribution(4.5/100, 2.5), 1/(2*(N+1))+1/(N+1)*i, numeric) + 0.068
Note 1 : with Maple 2015 (Windows 7) your code returns me a "connection to server failed"
Note 2 : the sequence [seq(q(i), i=1..(N-1))] runs very quickly : using Threads does not seem necessary
Note 3 : the CDF is a non decreasing function of its argument, and even here a strictly increasing one. As a consequence there should be only one solution to the "solve( (1/(2*(N+1))+1/(N+1)*i)=cdf(x),x)" command of yours.
In practice, for a reason I do not know, this command returns many solutions, some real and others complex (probably the reason for which you load the RealDomain package ?).
Using "Quantile" prevents these spurious solutions.