Without knowing the true business objective of the campaign it is an impossible question to answer - if we talk about brand awareness then fair enough the ad stood out (whether for the right or wrong reasons) and therefore awareness of the product would have increased. If the objective was to sell more chocolate bars then perhaps the ad was not as successful as it could have been.
Cadbury's 'chocolate' is a combination of milk solids and sugar with
very little cocoa in the mix, therefore the product should never really
hero in the ad because the product is of poor quality. Added to that,
Cadbury's is a household brand, therefore there is a need to do
something very different to take penetration levels to a new high. In
that respect the Gorilla did a decent job.
If we look at weekly spikes and troughs on sales figures around the
time of start and end to the campaign this could be more interesting. But bottom line, advertising is only ever part of the marketing mix so a rise or full in sales cannot over a considerable period of months cannot be wholly attributed to an ad campaign.