I’ve used a number of GMB kits before and have a few GMB timing boxes now. in general I’d avoid them for long term vehicles, valuable cars, interference engines, etc. I’ll install them on rusted out beaters that have maybe two years left before the frame rots through.
Don’t get me wrong, they’re decent enough and don’t have high failure rates so you’d probably get by fine. but nearly every aftermarket kit has higher failure rates than Subaru OEM. GMB doesn’t have a magic supply of OEM cheap parts no one else can find. Those two piece tensioners are like $150, id wonder if that’s s knock off or rebuild and looks like OEM. When it comes to parts that will strand you, or interference engines, I prefer the highest quality you can get if they don’t cost much more anyway.
If someone begged me to install a GMB kit for them and they want to save a few bucks on your car i would but first I recommend to save a few dollars:
1. Subaru belt and pulleys only (no tensioner)
2. Subaru belt and the lower cogged idler (by far it is the one that fails) and I’ll inspect the the others. The other ones very rarely fail. If you can DIY they can be inspected in 2 years or 40,000 miles in less than an hour, they’re super easy.
Those two piece tensioners don’t fail, I’ve never seen it but I’m sure it happens sometimes. They were routinely reused and I’d even convert new Subaru’s to the old style before they got old. If yours is in good shape, the seal isn’t wet - It happens so rarely and you already have a water pump, just buy the timing belt and pulleys from Subaru. I’d almost rather keep the OEM original tensioner and not install a new aftermarket unless it’s OEM new (not rebuilt)
3. If they didn’t like those options I’d still install the GMB kit without much concern but to me there’s just no compelling reason to do so and depending on the car/person I’d tell them I’m not touching it if there’s issues ! Haha
That said - I only work for free and don’t charge labor, so it’s easier for people to accept the higher cost parts. If you’re paying labor then the higher Costs changes the decision making process.