ES110 actually is multitimbral. If you enable that mode, any of its 19 sounds can be placed on any of its 16 channels, as illustrated on page 44 of the manual at
https://www.kawai-global.com/data/manuals/ES110_EN_R103.pdf
So yes, you could use Reaper and address up to 16 of the Kawai's sounds at once, on separate channels.
There are, btw, many many boards that do not have GM compatibility (or even the ability to play 16 sounds at once, GM or not), including not just other ES-110-like digital pianos (i.e. competitive models from Korg, Roland, Yamaha), but even many higher end boards, including Korg SV1/SV2/Grandstage, I think most (all?) Yamaha CP models, and every Nord ever made, among others. Lack of these capabilities does not equate to being a home entertainment unit!
In fact, these days, I'd say GM is of more interest to home hobbyists than to professionals. And while "workstation" and "arranger" style keyboards are 16-part multi-timbral, probably the majority of boards sold as pianos, organs, or synthesizers are not, regardless of whether they are low end or high end models.