The Animorphs Wiki states that
Because they are slow and inefficient, the Gedds formed a symbiotic relationship with the Yeerks, who helped them survive in exchange for being hosts.
How exactly did the Yeerks help the Gedds survive?
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The only instance I can find in the books of anyone actually claiming that the Yeerks and the Gedds had a symbiotic relationship came from Prince Seerow, shortly after being informed of the Yeerks' first attack on the Andalites. As the greatest advocate for the Yeerks, Seerow had much to lose from this sudden but inevitable betrayal, and it's reasonable to believe he may have stretched the facts just a little in his desperation to defend the Yeerks and/or his decisions.
<They gave you their word?!> Alloran practically shrieked. <Their word? They're parasites. The Yeerks steal the bodies of other species. What did you expect of them?>
<They have no history of harming intelligent life-forms. The Gedds are barely conscious in their natural state,> Prince Seerow argued. <It's not as if they were stealing the bodies of truly sentient creatures. They and the Gedds are symbiotic. They have ->
Alloran stepped closer to my father. <Listen to me, my prince.> The word "prince" was said with a sneer. <Approximately four hundred Gedds attacked our ground base last night. They overwhelmed the two dozen Andalite warriors on duty there. The two dozen Andalite warriors who had been specifically ordered not to fire on Gedds.>
Hork-Bajir Chronicles, pg 7
Further, there are also indications that the Gedds resisted their infestations to what small extents they could.
When it was my turn, the Gedd's head was thrust beneath the surface of the pool.
Hork-Bajir Chronicles, pg 21
I felt a resistance. A mind within the brain. Stronger than the tired, beaten Gedd. This [Hork-Bajir] was attempting to fight me!
Hork-Bajir Chronicles, pg 33
By the time I infested my first Gedd there was already a huge body of information on how best to subdue the Gedd mind. The same when it was time to move up to Hork-Bajir.
Visser, pg 32
Still, as the Wiki says, it's stated numerous times that the Gedds are barely sentient and physically awkward. While it's possible that they form an approximation of a symbiotic relationship through something like the Yeerks granting them their intelligence, such a situation is never stated outright, and the Gedds seem no more willing to be Controllers than most other races.