I think there are many physical, emotional, and official reasons for Gai not being an active ninja anymore.
Let us first consider the physical. Gai opened all of the gates and launched an attack that had great backlash against his body. This can be compared to Lee opening 5 of the gates and using the ura renge. Lee had spinal damage, nerve damage, and muscular damage, so we can assume that Gai had at least all of this, and definitely more. His body was also on the verge of all turning into ash. Naruto was able to stop this reaction from affecting his body as a whole, but we can only imagine the internal damage that he must have received. It can be assumed that it was more than Tsunade would be, in good conscience, willing to operate on (Lee's lesser injuries had an incredibly low survival rate, so imagine Gai's.) Missing most of your toes also does not require a full leg cast like that. We must remember that he also broke just about every bone in that leg, in ways that could not possibly heal correctly. Physically, Gai's ninja career was over.
Next, emotionally. Gai fully committed himself to his final attacking being just that: his final attack. He did not intend on surviving the technique, and in a sense passed on his will to Lee and the future young leaves of Konoha. His father, Dai, also died after passing on the technique and conditions to Gai, and Gai's most defining trait is the strictness of his own rules. In his mind, his life was forfeit the moment he opened the gates, and surviving and continuing to actively fight after that would be breaking his own rule of throwing his life away for the next generation. He has entrusted everything to Lee, so coming back and effectively taking back all that he entrusted is not in his character.
Lastly, the official reason for Gai not being active is that, as a Jounin, he does not NEED to be active. Historically (or rather, allegedly) Jounin were the ones that handed down the orders from the top of the ninja chain, with Chuunin being the squad leaders of genin, the genin being the ones who actually did grunt/wet works. Gai is still in the highest ranks of the Konoha military system, despite not being a front line fighter. This was demonstrated in the movie when he was in uniform beside Kakashi directing the taijutsu assault against the incoming meteor. Gai can lead Chuunin and genin from his (wheel)chair, like upper ranking military do.
What do you think?