It's technically (I say "technically", ask an economist and they will have to admit it is "very definitely") a net gain because they are selling a service and people are paying for it. The GDP is the total sum of all goods and services sold and this hole will increase that total by a small amount. I think the hole digging is hilarious and very clever. :)
GDP is gross domestic product. The problem is in the term "gross": GDP doesn't measure depreciation.
Arguably here they're pumping $X per day into a project that is also depreciating at $X per day (i.e. no net value is produced, ignoring the entertainment value of the stunt). So the project contributes to GDP but not to net wealth.
The failure to account for depreciation is one of the major problems with GDP as a measure of prosperity.
