Someone has resurrected a dinosaur in digging up this old thread! Yep, I'm still running this monster G thru the southern deserts and Rocky Mountains.
I've had the Tibus Portals on the heavy (7000lb +) and slow (60mph max) camper for around 15,000 miles or so. 90% of those miles have been off-road. Lots of forest roads, lots of slow, washed out rough rutted trail, crawling as needed and plenty of mud with some occasional deep water thrown in.
So far the portals have been perfectly reliable thru all of the abuse I've put them thru. They run nice and cool.
I have had an issue with the rear brakes producing excessive heat and ended up replacing the entire original (circa 1981) disc/drum braking system with disc/disc 463 parts - there are quite a few different between those two systems including volume of the pressure booster, master cylinders and you should remove the rear check valves and porportioning valve. Not addressing these differences in proportioning can allow the brake caliper to heat up enough to soften the outer portal seal and you'll get a leak - that's what I did. The Tibus kit comes with disc brakes front and rear but at the time I installed mine there was zero guidance on the rest of the braking system mods - I had to figure it out but now it makes sense. BTW, once sorted these brakes rock.
I wouldn't suggest to anybody that this is an easy DIY installation and if done that way I wouldn't be surprised if there were problems.
Instead, I'd suggest an experienced master mechanic who is capable of precision portal alignment (it has to be perfect), familiar with gearboxes of various types and is intimately familiar with G Wagen axles - should be able to rebuild one to factory specs while blindfolded.
The GFG ones are likely the LeTech portals, which are the ones used on the 4x4 squared. They are things of beauty but like you said they are big bucks. Are they better than Tibus, I'd bet they are.
The performance advantages of adding to portals to a vehicle are nothing short of astonishing. I would have had to put on 44" tires to achieve the clearance that I have with 35's and portals. Needless to say I'm a fan of a portal G of its done right.
Hope this helps.
William