I can not say if there is any canon about the specific combination of gnoll and cleric of Erythnul. However, a Pathfinder third party publisher has a large page regarding gnolls, where they state the following (emphasis mine):
Gnolls treat their dead like they would any other dead creature and
engage in cannibalism, but they may offer a brief ceremony for a
revered member of the pack prior to devouring the fallen gnoll.
Additionally, if we look at the wiki on Erythnul it states that (emphasis mine):
In the least violent services to Erythnul, shrill reed instruments are
played discordantly while gongs clash and drums pound. During major
rites, a fire is built and victims are sacrificed. One famous rite is
the "Bloody Howl," when soldiers captured from the previous battle are
killed in order to bring Erythnul's favor just before the next one.
Prayers to Erythnul are customarily rhyming chants with gory subject
Combining the two, I can imagine 2 scenes (depending on what is available):
Scenario 1 (No prisoners):
You (as the cleric) would construct a funeral pyre and burn your party
member (to perform something as close to the 'major rites' that one
can get). You could then take part of the roasted party member and
consume his flesh (to appease the Gnoll side of you).
Scenario 2 (Prisoners were taken at whatever incident killed your party member):
You would herd the prisoners into a small encirclement, where you
would burn them alive, accompanied by either chanting or instruments
(drum/gong). You would then at some point consume part of your party
member before adding the remainder of his corpse to the burning area.
PS: I never realized how... 'savage' Gnolls can be before trying to answer this question.