It happens naturally once in a while, so if you just try hard enough, it will eventually also happen with a bit of gamma ray. But the gamma rays won't do any good!

If you want to induce polyploidy, use Google to learn that colchicin is what is commonly used for that.

Numerous biological parameters such as genotype, genome, ploidy status, chromosome length, DNA content and nuclear volume are now known to condition the response of plants to treatment with radiations. Among these, the effect of ploidy is of particular economic are polyploidys and since both the nature and extent of polyploidy influence radiation sensitivity there by redendering response to radiation as a useful tool in the analysis of the nature of polyploidy in a given plant. For more details consult https://www.tandfonline.com or Int. J. of Cytology, Cytosystematics Cytogenetics Vol. 21: 4, 303 - 310, DOI: 10.108000087114.1968;10796308

Polyploid also may occur due to abnormal cell division, either during mitosis, or commonly during metaphase I in meiosis. In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. So it can be inducted by x ray irradiation as well but you cant exactly control the process.