Don't necessarilly count on the seeds all being "giant" as well. Any basic genetics course will tell you this:Even when a plant is self fertilized, genetic recombination occurs (as well as possible mutation). Depending on the traits of the "giant gene", this may leave you with all "normal" plants, a mixure of normal and giant, or all giant.
The point is, without studying the heredity of the gene responsible you just don't know (unless someone has shown it to pass on to the offspring with high fidelity, and even then I wouldn't count on it 100% due to all the genetic craziness that happens in the process sexual reproduction)... If you ever give away the seed, labling it as "seed from capensis giant form" will suffice. Just remember, SELF FERTILIZED PLANTS ARE NOT CLONES!