We can take different approaches to answering this.
Trying to analyse it, I have to conclude that since et al means "and others" that I would treat it as plural just as I would if I'd simply used the English "Gamma and others are saying" rather than "Gamma and others is saying".
We can look at patterns of usage. Here this look at google ngrams suggests the use of the singular. However, it's close and neither are very common. This look at ngrams uses more possible verbs, and finds that generally the plural is more heavily used, and much more with "...et al have" vs "...et al has".
So far it would seem that we should definitely use the plural. A counter-argument is that we often use the author or authors of a text to refer to the text itself. If this is the case, then it should be treated as plural. However, that doesn't seem to be the usage here, and indeed we could say that this may explain some of the singular use found by examining ngrams above, so in fact it actually adds weight to the argument in favour of plural use. It does suggest that the following are both correct though:
"Gamma et al are saying..." referring to the authors.
"[Gamma et al] is saying..." referring to the text, where the bibliography has an entry of that and for some reason we prefer not to just list it as "[Gamma]".