Most gerunds are regular, but a handful adjust — always in one of three ways:

Change	Infinitive	Gerundio
i → y between two vowels	leer, oír, traer	leyendo, oyendo, trayendo
e → i in some -ir verbs	pedir, decir, vestir, divertir, reír	pidiendo, diciendo, vistiendo, divirtiendo, riendo
o → u in dormir, morir and poder	dormir, morir, poder	durmiendo, muriendo, pudiendo

Ir has the odd form yendo, but you'll rarely need it: for I'm going, Spanish usually just says voy.