The sentence is a bit complicated because it part of a dialogue, and Gargamel is not saying some words which are only implicit:

• Gargamel ! Lâche mon robot ! (Gargamel! Let go of my robot!)
• [Oui, Gargamel] lui-même, pauvres naïfs ! (Yes, Gargamel himself! Poor gullible fools!)

What Gargamel is replying is first that yes, it’s Gargamel himself (“lui-même”) who is speaking. He is following up on the sentence of the first Smurf, who just noticed Gargamel and cried his name in shock.

Then he is calling the Smurfs “pauvres naïfs”. I assume he is speaking to several Smurfs (perhaps including the robot in his hand?), otherwise it would not make sense to have a plural here.

Here, both “pauvre” (poor) and “naïf” (gullible) are adjectives; but he is using the adjective “naïf” as a noun, a process called nominalization. Thus “un naïf” is, well, a person who is “naïf” = “gullible”. In the same way, in French we can say “un riche” (a rich person), “un pauvre” (a poor person), or even “un Français” (a French person). So while “naïf” is originally an adjective, here, it is used as a noun. This is hard to translate in English, where such a construction is rarely possible, and you can see above that I had to apply some creative leeway – adding the word “fool”.

Finally, you have the word “pauvre”, “poor”. It is indeed an adjective, qualifying the noun “naïf”. In this context it does not mean someone who has no money, the primary meaning of “pauvre”; rather, it is used to intensify the negative “naïf”. It means that the Smurfs are not only gullible, but their gullibility even inspires pity.