Can someone cite an example of incomplete dominance with gain-of-function mutation?
As a trivial example, one could use the epigenetic silencing of transformed reporter genes.
Briefly: Genetic constructs are transformed into some organism (in this case, plants). These constructs usually include a "reporter" gene that is expressed at high levels, which allows workers to artificially select successful transformants. Examples might be GFP genes, metabolic genes complementing auxotrophies, or antibiotic resistance genes, all of which would be clear examples of a "gain of function" mutation when integrated into the genome.
After one or more generations, these reporter genes often get "turned off", in that their expression decreases dramatically or ceases entirely (this tends to happen most when the reporter is expressed very highly or is present at high copy number). I'd suggest looking into the linked paper for more information.