Apparently...there is a Gamera Collection coming in 2020
The December 2019 issue of Home Cinema Choice (UK) contains an interview with Fran Simeoni from Arrow Video, and he states:
"One of our big upcoming projects is a collection of Gamera films; many of these will be available for the first time in the UK and it’s going to be a real treat to see them all packaged together, with loads of extras and stunning packaging. We love working on boxsets and this is one of a few we have planned for 2020, including our [Alejandro] Jodorowsky collection."
It is unknown if this collection will be UK exclusive or which titles will be included in the collection.