When traveling overland, the referee rolls 1D6 once every 12 hours to determine if an encounter occurs. A result of "6" indicates an encounter with some type of creature or being, as determined on a separate 1D20 table appropriate to the terrain. How such creatures/beings react to the PCs is left to a combination of the reaction table and referee adjudication. In fact, it's worth noting that the text in this section of the rulebook makes no reference to the reaction table whatsoever. Whether that's a mere oversight or indicative of the fact that the game had multiple authors and poor editing I cannot say.
I will admit now that I have always loved Gamma World's menagerie of creatures, which I consider to be every bit as interesting as D&D's own, perhaps moreso as they're almost all wholly original creations. The rulebook provides very little in the way of mechanical information about each creature, with their names, number appearing, armor class, movement, and hit dice being the only standard information offered. While this makes for compact entries, it can also be frustrating at times. For example, knowing what mutations a creature possesses at a glance would be helpful, as would knowing how many physical attacks it gets and the damage dealt. Likewise, Gamma World is much more mechanically dependent than D&D on ability scores, particularly Mental Strength and Constitution, most creatures have no such information in their entries. It's in this area where the second edition excels, in my opinion.
Another possible flaw, depending on how you view it, is that, of the nearly 50 creatures described, only one -- the yexil -- is given an illustration anywhere near its entry. A handful of others, like hissers and hoops, are given illustrations elsewhere. I think this is potentially an issue primarily because some of the creatures in Gamma World are bizarre enough in appearance that the lack of an illustration makes it hard to imagine what they look like. When you couple this with the fact that most creatures have names that don't in any way call to mind what they are I think it could become an issue for some. Of course, it was never an issue for me personally, but I readily admit I'm a weird obsessive about such things and can, even now, tell you what a parn is and how it differs from a zarn.
Overall, I like Gamma World's selection of basic creatures. I think it covers most of the obvious bases, providing a good mix of humanoid and animal mutants, both malevolent and benign, as well as mutated plants of various sorts. I'll admit to being especially fond of androids, perhaps because they played a big role in one of my old campaigns. I also like badders (evil badger men), centisteeds (16-legged horses mutants), hoops (rabbit men), orlens (2-headed, 4-armed mutant humanoids), sep (land sharks), and serfs (which I always played like the mutants from Beneath the Planet of the Apes). But, truth be told, I don't have a problem with any of Gamma World's creatures, though there do seem to be an inordinately large number of fish and water-based creatures for my taste.
Interestingly, I always found it much easier to create new creatures for Gamma World than I ever did for D&D. A big reason for this is that Gamma World provided a large list of mutations I could add to a real world animal, plant, or human being and -- Voilà! -- a "new" monster. In truth, D&D monsters are not harder to create, but, for whatever reason, those lists of mutations was a huge boon to my imagination and I used the heck out of them, proving once again that randomness is awesome.