Stacy...
I don't doubt for a minute that Gemini suggested or recommended doing what you posted, however, in their catalog it clearly shows how to mount a letter to a corrugated wall. It's what sky had mentioned and I eluded to about questions over the last few months.
Please understand... this is not meant towards you or anyone in particular..... just the general trend that seems to be creeping into our industry and is becoming huge topics of late.
Again, Gemini will tell you whatever you want to hear in order to finish a sale. As you said in your own post, if you can't do it, the next guy will. Same goes for Gemini. The next guy might make his own letters or buy from one of their competitors, so they will give you anything to make you happy. That does not mean it's the preferred or best way to finish off a job.
In addition, there isn't ONE installation picture of the method you described anywhere in their catalog..... at least 2010. Do they sell the parts to do it ?? Of course, because there are far more people doing this stuff, not capable of selling and/or doing it the correct way. They need to be prepared.
During the Atlantic City Sign Show this past December, I was talking to my Rep who has a large territory. They've been downsizing like crazy and still meeting their deadlines. Dick said the reason for this is because of the CNC sales and people doing it themselves to cut them out. They are scrabbling to make ends meet. If it means selling you a few tubes of silicon to get the job done.... it beats not making the sale at all.
Again, don't take this personally. I was not speaking of you, but you were honest enough to post it and it shook my brain cells a little to rattle this off. Its not meant to hurt. If it did, I apologize.