Are you kidding me? Gustavo’s entire character arch is about getting revenge for the murder of his boyfriend by Hector Salamanco! It gets explored further in Better Call Saul as well.

It was subtext in the original series, but not explicitly said. I think there was a scene in BCS that did make it explicit though.

It was subtle, but relisten to his threat from the Crawl Space episode:

I will kill your wife.

I will kill your son.

I will suck off your brother in law.

I will kill your infant daughter.

It is implied that he had a 'special' relationship with his partner when he was first starting his business and working with the cartel. The cartel killed his lover in front of him to send a message or something. Idk. It's been awhile.