Molecular phylogenetic studies (Joly et al., 2017; Martén-Rodriguez et al., 2010; Watson 2015) have mostly supported a sister-group relationship of Gesneria and Rhytidophyllum, but taxon sampling is limited and statistical support is low. A more comprehensive taxon sampling to evaluate Gesneria and Rhytidophyllum is an ongoing collaborative project by several botanists (e.g. Joly, Clark and Martèn-Rodriguez).
Several morphological characters reflect a separation. The most useful character to differentiate Gesneria and Rhytidophyllum is the presence or absence of fusion of filaments to the corolla tube. The term connation is used to describe similar parts that are fused (e.g., the fusion of petals to form a corolla tube as in most members of the Gesneriaceae). The term adnation is used to describe the fusion of dissimilar parts, such as the fusion of filaments to the corolla tube. The character that defines Rhytidophyllum is the presence of filaments that are adnate for 2-5 mm to the base of the corolla tube. The character that defines Gesneria is free filaments or complete lack of fusion (i.e., absence of adnate filaments to the corolla tube).
The illustration below features an open flower of Rhytidophyllum crenulatum (top) where you can observe a zone of adnation for 2-3 mm near the base of the corolla and a zone where the filaments are free from the corolla tube. These two regions (e.g., separate filaments that are above the zone of adnation) are indicated on both sides of an opened corolla tube. In contrast, the filaments in Gesneria (bottom) are free or separate from the entire length of the filaments and therefore have no zone of adnation.
Another character that is useful for differentiating Rhytidophyllum is clusters of barbate (bearded) hairs at the base of the filaments and immediately above the zone of adnation. In Gesneria, the base of the filaments is glabrous (=without hairs). Barbate clusters of hairs are indicated with white arrows or lack thereof.