I doubt they are intentionally ignoring you. Many amateur equipment vendors, including GAP, are very small operations and have nobody actually checking their e-mails except perhaps once a week, if they have time.
I've owned GAP vertical antennas as well as others, and they are "all" affected in a similar manner by the presence of metallic objects close by. Best situation is, if you can locate the antenna so that it's at least 20 feet from the stucco, you'll be far better off. I had a GAP Challenger DX-VI (31-1/2' tall, 80-10m) vertical installed about ten feet from a wrought iron fence (only 5' high) and it was rather horrible and non-resonant in any of the ham bands, until I tried moving it another 10' farther away (now 20' from the same fence). What a difference! Night and day. Works fine, perfectly resonant, no tuning required.
Based on this experience, I'd say it's obvious, "the farther, the better" when it comes to proximity to anything, especially anything metallic such as the wire lathe in stucco.
73 de Steve WB2WIK/6