I have gastroparesis (GP) that the doctor described as "mild, but not insignificant."
Basal rate testing for most people doesn't start until four hours have past since the last meal. The idea is to isolate the BG rise from food from the BG rise driven by the release of gylcogen from the liver. The ideal basal rate will metabolize liver sugar only. The four hour set-up period allows both the food and the last meal/bolus insulin to completely play out.
For someone with GP, it may be necessary to extend the basal testing set-up period, usually four hours, to a longer period, maybe eight hours. I know that this is hard and inconvenient, but discovering your actual basal needs will make your insulin dosing easier. I've always found extended fasting, beyond eight hours sleeping, makes it easier to control my blood glucose, not harder.
What I would do is eat a meal that you know that your GP tolerates well and then do the 4 hour set-up followed an initial fingerstick BG. I would test every half hour to monitor. What you want is a flat-line BG that does not vary by more than 30 mg/dl from the initial fingerstick. If your BG is rising more than 30 from the start then either your last meal is still digesting or your current basal rate is too small.
This is the quandry that makes basal testing with GP more difficult. At this point, I would corrrect your BG with glucose tabs or insulin (hopefully these will be minor corrections) and add four more hours to the set-up period. If, at the end of the second 4-hour set-up your BG is still rising or falling more than 30 mg/dl, then I would abort that test and try another day.
By the way, I use intermittent fasting as part of my way of eating. Since I limit the number of carbs I eat every day, currently to < 50 grams/day, I find that I don't get hunger pangs and makes fasting easier. What I do now is to limit almost all of my eating to between 11:00 a.m. and 7:00 p.m. which, in effect, produces a 16-hour fast every day. I like what it's done for my BGs.
In summary, basal testing for someone with GP may be harder. Adding a second consecutive 4-hour setup period may be what you need. As I said before, knowing your actual background insulin needs will make your insulin dosing easier and much easier to analyze. Basal testing is a pain but the benefits exceed the work.