Before discussing the details of what would be involved for a dancer who already knows Mainstream to learn GDP and vice versa, it is important to understand that this issue would come up with any list that isn't widely used. If we adopted the ACA "One Floor" program, or the Rio Grande Valley "Club Level 50" list, as a way to make square dancing easier to learn in our area, the same kind of problems would arise. The developers of those lists hoped and have advocated for them to become widely used, but in fact this has not happened. So the chances that somebody visiting or moving from an area using one such non-standard list to another would find the same list is pretty small.
It's also important to understand why this would seem to be a new issue for us. Dancers who learn in this area don't have this problem when they go elsewhere because we currently teach everybody Plus. All of the smaller lists, including Mainstream, are subsets of Plus. However, in the other direction we currently do have this issue but we think about it in different terms. If a dancer who only knows Mainstream and has been happily dancing Mainstream comes to our area, he or she simply won't find anywhere to dance on a regular basis. We just dismiss this with an "oh well, they don't know Plus". But there is no practical difference between such a dancer and one who might come from an area where people were happily dancing the ACA or Rio Grande lists -- they would all find themselves in a situation where the gap between what they know and what we dance (Plus) is so large that they (and we) don't even attempt to bridge it on a short term basis or make any other special provisions for them. If they are moving here permanently, they just join a Plus class.
Introducing GDP into our area won't be creating a new problem for anybody who is currently dancing either Mainstream or Plus. Yes, it will create a new category of dancers (those who have learned only GDP) who won't be able to dance elsewhere -- but almost all of those people wouldn't be dancing anywhere without GDP. And the people coming from Mainstream areas will have a problem that seems like it is due to the differences between Mainstream and GDP -- but without GDP they would have faced the even greater problem of the differences between Mainstream and Plus. If they are moving here permanently they will probably still want to learn Plus, but having GDP will actually make it easier for them to start dancing in our area, because they will have a place to dance without learning all of Plus, while they are learning the rest of Plus.
This is probably too much for an average visitor to want to learn just to dance one dance, but it's possible to imagine giving a particularly smart and enthusiastic visitor a "crash course" on these calls in an hour or so before a dance, which would probably be enough to allow them to survive. And while the migration rate into our area from Mainstream areas is nowhere near high enough to justify scheduling anything on a regular basis, if even a few people from Mainstream areas happened to move here around the same time (perhaps even just one family), it might make sense to arrange a special class for them. Either a few evenings or one weekend day should be enough time for such a "class" to bring competent Mainstream dancers up to the GDP level.