If the conjugation adds an 'e' to the stem, then the 'g' is replaced by 'gu'

castigar - to punish

abogar - to plead

abotargarse - to swell up

abrigar - to wrap up, to keep warm

abrogar - to repeal

agregar - to add

bogar - to row

arrepanchingarse - to stretch out, to sprawl

alargar - to lengthen

aletargar - to make drowsy, to send to sleep

amagar - to show signs of

apagar - to put out, to put off, to extinguish

apechugar - to put up with, to live with

chocar - to crash

apegarse - to become fond of

arraigar - to establish, to take root

catalogar - to catalog

arriesgar - to risk, venture, suggest

congregar - to congregate

derogar - to repeal, to rescind

desahogar - to vent, to relieve, to ease

desarraigar - to banish

desarrugar - to smooth out, to iron

descabalgar - to dismount

descargar - to unload, fire (pistol)

desligar - to untie, to separate

despechugarse - to bare one's breast

divagar - to digress

embriagar - to intoxicate

excolmugar - to excommunicate

fatigar - to tire, to weary

halagar - to flatter

instigar - to instigate

juzgar - to judge

largar - to pay out, to give

litigar - to go to law

llegar - to arrive

madrugar - to get up early

prodigar - to lavish

sobrecargar - to overload

tragar - to swallow