The non-migrating chicken is different from the migrating goose; force-feeding will not result in a liver with a significant higher fat content. The chicken liver plays a role in producing fat but not in storing it in itself.
The adipose fat in chicken is mainly in the subcutaneous (under the skin), abdominal (stomach) and clavicular (shoulders) region (study looked at ages 4 and 14 days only). See also another study which is bit broader.
Alshamy et al.  found no adverse effects to the livers of chicken under a high energy diet. They compared two different chicken lines, a dual-use line (Lohmann Dual) and a broiler (Ross 308). The Lohmann Dual had 9% more lipids (fat) in the liver than the broiler.
Average fat content in chicken liver (the Korean study I found says "Ross breed" which are broilers but there are many Ross breeds) is about 2.9% for that line. As this is a Korean study it might be interesting to know that the fat content as listed in the USDA is 4.8gr per 100gr (https://fdc.nal.usda.gov/fdc-app.html#/food-details/171060/nutrients).
Geese on the other side need reserve for migration. That reserve is fat and it is mainly stored in the liver.
Fat content of Foi Gras (liver of force-fed geese) according to the USDA is about
44gr per 100gr (https://fdc.nal.usda.gov/fdc-app.html#/food-details/171100/nutrients). The product listed in the USDA is canned smoked Foi Gras*. Another, french, source (https://www.lanutrition.fr/bien-dans-son-assiette/aliments/viandes/le-foie-gras-et-votre-sante) has the fat content of raw goose liver ("Foi Gras d'Oie" in French) at ~55gr per 100gr if force-fed.
There seems to be a small movement to use the naturally fattened liver by slaughtering them at the times when they prepare for migration. I can only give the source I found in Wikipedia that a Spanish company won a French price (https://web.archive.org/web/20071128130319/http://www.regiondigital.com/modulos/mod_periodico/pub/mostrar_noticia.php?id=47071) for their Foi Gras produced without force-feeding. An animal friendly (besides the slaughtering) Tournedo Rossini? Mmmh, I wish I could afford that!
If you are not familiar with the recipe for Tournedos Rossini: it is basically a thick slice of roasted Brioche topped by a nice tournedo topped by a thick slice of Foi Gras topped by a thick slice of black truffle (Perigord truffle, Tuber melanosporum) topped by a generous ladleful of Madeira sauce.
 Shiping Bai et al. "Broiler chicken adipose tissue dynamics during the first two weeks post-hatch", Comp. Biochem. Physiol. A Mol. Integr. Physiol. 2015 Nov;189:115-23, DOI: 10.1016/j.cbpa.2015.08.002 , abstract at https://pubmed.ncbi.nlm.nih.gov/2626385
 Z. Alshamy et al. "Structure and age-dependent growth of the chicken liver together with liver fat quantification: A comparison between a dual-purpose and a broiler chicken line" in PLoS One 2019 Dec 27;14(12):e0226903.. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0226903
 A. M. Fouad et al., "Nutritional Factors Affecting Abdominal Fat Deposition in Poultry: A Review" in Asian-Australas. J. Anim. Sci. 2014 Jul; 27(7): 1057–1068. at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4093572/
 Guosong Wang et al., "Transcriptomic analysis between Normal and high-intake feeding geese provides insight into adipose deposition and susceptibility to fatty liver in migratory birds" in BMC Genomics volume 20, Article number: 372 (2019). at https://link.springer.com/article/10.1186/s12864-019-5765-3
 Pil Nam Seong et al., "Characterization of Chicken By-products by Mean of Proximate and Nutritional Compositions", Korean J. Food Sci. Anim. Resour. 2015; 35(2): 179–188. at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4682518/
* Smoked? Really?