The G.U.M. Drops on Rainbow Resource does not appear to be the same G.U.M. curriculum by Zaner-Bloser. We used the G.U.M. Drops by Melissa Schneider from RR for two years and loved it. It is Fix It Grammar style (editing passages) only a little gentler, so my daughter used it for 2nd and 3rd grade and moved into Fix It by IEW this year for 4th grade. From what I’ve read since starting to teach this subject in our homeschool, teaching grammar at a young age (early elementary) is difficult as kids’ brains are not wired up yet for the thinking skills required for learning grammar rules. This is why it many try it in the early years but don’t see any progress/fruition until the upper elementary/middle school grade levels. That said, another program you might be interested in considering is Treasured Conversations. 
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