Today I finally got the Gaplant, and it is EXCELLENT! I like how big it is, the transformation is clever and sturdy (though I wish there were tabs to snap the legs together), the colours are good, and the pipe detailing is also nice. You can swoosh it around pretty easily too! I also like how it had the same red thrusters as the Raider; that's particularly soulful and toylike. Older kits seem to be pretty soulful in that way, and I enjoy that.
Also, what is the big issue with transforming the RG Zeta? I really want a transforming Zeta, but Wave Shooter is too expensive and hard to find (is there any chance it will be reprinted at some point?), and the RG looks really cool and detailed, so I'm kind of leaning towards the RG. Complex transformations aren't an issue for me, so how complex is it compared to, say, a Transformers Bayverse Studio Series figure?
From what I've heard about it, it suffers a bit from early RG issues like the use of the Advanced MS Joints that are compounded by needing the space for the transformation gimmicks. The connections and joints could loosen up over time, so you might end up having to snap parts back on while you're trying to transform it or have difficulty making it hold a pose. Still, if you're looking for a fully transformable version of it in that scale, it's your only option, so it does make the buying decision easier. Take a look at
Dalong's review of it to get an idea of how it all comes together, he documents the entire transformation process as well as various quirks of the kit. If you have the space for a MG Zeta, especially
the Ver.Ka revision, that might be worth looking into, though I suppose that wouldn't be the same scale as your Gaplant.
As I mentioned before, RGs have gotten better over time, and newer kits tend to be sturdier and better about their gimmicks. RG GaoGaiGar surprised me with how solid the finished product is after transforming and combining all the mechs together, with only a couple fiddly connections. Earlier kits aren't bad, they just hadn't been developed as much yet, and they're still some of the best looking kits in 1/144 scale.
And yes, everything does get reprinted eventually, but sometimes it'll take a while, especially P-Bandai kits like the Wave Shooter. I did a search to see when it was last reprinted, but unless I'm mistaken, I don't think it was ever available on US P-Bandai, so that would explain why it's hard to find here. Looking at other regions, it seems like it was only printed once in 2017, so it's definitely been a while. For kits you really want, it's a question of how patient you're willing to be versus how much money you're willing to spend.