How to make a gel battery to retain its capacity again?

It's unlikely that it can be revived.

12 V 5 Ah is very small for anything with a starter motor. You would almost certainly do better with a larger battery if you have any place to fit one.

Gel batteries left unused or in "standby" applications like security alarms of UPSs can sometimes be revived to some extent, using a pulse "desulfator" as the plates can develop a sulphate coating when they are not being cycled to some extent. That can be broken down by electrolysis, if the battery is otherwise OK.

But, a gel or wet lead-acid battery in regular high-current uses tends to degrade by the plates cracking and loosing material, so there is no way they can be revived, it's an irreversible process.

My riding lawn mower had a gel battery that did not last very long. Replacement gel battery was $187. A small lead acid battery several times larger than the gel battery was $25 at Walmart. Lead acid battery is larger than the gel battery but I made it fit in the space available and stay in place with 2 straps. So far lead acid battery has lasted 3 years, I keep a tiny 1 amp trickle charger on the lead acid battery all winter.

If the battery is more than a few years old, it likely can't be revived.

First, if it ever worked starting that motor, make very sure all contacts/terminals are very clean and tight. The best friend of a battery is a wire brush.
Any lead acid battery that is left discharged over 24 hours will never recover its original capability. Buy a wet lead-acid that fits the housing and never leave it discharged.

I join the chorus of what other people have said: lead acid batteries, whether gel or flooded, can sustain significant abuse. However, left in an uncharged state for a more than a day or so, and the plates will degrade. Mostly irreversible.

I had some sealed lead acid batteries from the UK that were completely discharged and then left in a container for 3 months and then still packed for another year. Totally useless. I used them to power an electric fence module (charged every day) for a little while. At first they lasted around 1 day but as used more they ended up lasting about 1 month. I believe that desulphators work.

Mike.