Hi,
b) Migrate and use Alpha2 in Read Only Mode
Note: Alpha2 is only for testing. There is no possibility to update Alpha2 to the next version.
If you use the Alpha2, you would have to deinstall it later and perform the migration again.
A) Migrate but not yet activate
I think we can cope with Joomgallery not beeing available for some time
If you can do without it until the release of JoomGallery 4.0, this would be possible.
Personally, however, I would do the following:
Create a copy of the Joomla 3 site with JoomGallery 3.6 and keep it running in the background.
Update your regular site to Joomla 4. Leave JoomGallery 3.6 deactivated there.
After JoomGallery 4.0 has been released, perform the migration of JoomGallery.
Regards
MrMusic