From Raintree: Gisela 3 is the most dwarfing of the Gisela rootstocks making a tree that grows to only 8 to 10 feet tall. It tends to make a broad tree excellent for a small area. Its small size and early heavy bearing are great attributes but because of this the tree needs good growing conditions to thrive. It is very precocious prompting the tree to bear heavily at an early age. It may require fruit thinning to maintain fruit size and avoid overbearing and having the tree stop growing. Regular irrigation is needed. It is recommended that dormant pruning on all dwarf cherry trees be done in late winter before bloom time which reduces the chance of bacterial canker infestations.
So I am not sure about low productivity, unless we talking about productivity per tree, not per sq or cu foot taken by the tree.
Ok, I found the study you mentioned:
In a rootstock trial set up in 2002, trees of Regina, Karina, Kordia and Merchant sweet cherry were planted at different within-row spacings on Gisela 3 (1.5 m, 2.0 m, 2.5 m) and Gisela 5 (2.0 m, 2.5 m, 3.0 m) rootstocks. Distance between the rows was always 4.75 m. Preliminary results are given after seven cropping years. All cultivars consistently produced a higher yield on Gisela 5 than on Gisela 3. In the first years, there was no clear effect of planting distance on yield per tree. The highest yields per hectare were obtained most frequently with Gisela 5 at a planting distance of 2.5 m. Even the highest density of trees on Gisela 3 (1.5 m) did not exceed those yields per hectare. Tree survival rate in the 8th year on Gisela 5 was 100% with Regina and Merchant, and approx. 90% with Kordia and Karina. Merchant and Regina were also the healthiest cultivars on Gisela 3 with tree survival rates of 92% and 71%, respectively. Survival of Kordia on Gisela 3 was also 71%, while that of Karina was only 53%. Summarizing all experiences under Northern German conditions, Gisela 5 was the best rootstock for the most relevant varieties. Gisela 3 is currently being considered especially for high density systems and plantations under rain cover protection. However, our findings of low tree survival rates on Gisela 3 indicate a high risk for growers.
Unfortunately even 2.5 sq meter per tree is luxury I do not have in my yard, forget about 2.5X4.75. So in this case I have to compare smaller crop of G3 with crop of HIGHLY pruned G5(to fit the spot). I am not sure if G5 will win in this case.