Gerardo,
gensim doesn't contain pLSI, mainly because pLSI is fully superseded
by LDA. With uniform topic priors, pLSI and LDA use essentially the
same model, so with enough training data, you'll get the same
results.
But an added advantage of LDA (apart from more flexibility w.r.t. non-
uniform priors) is that you can infer topics for previously unseen
documents, which you can't with pLSI.
HTH,
Radim