I need help with the backstory of my half-Goliath fighter, Gorden. He is a rune knight that specializes in unarmed fighting and grappling, and he has the Entertainer (Gladiator) background. Life events say he fell in love, made 6 gp working, kidnapped someone and was imprisoned for 4 years, and made an enemy of a lawful evil cleric. I was thinking that he was taught rune magic by a giant that he somehow impressed. Anyone have some ideas I could use? 

First thing you have to decide is what kind of character do you want Gorden to be at the start of the campaign.  Is he fleeing from something terrible?  Does he have a tragic past he wants to escape?  Is he looking for redemption? Is he a happy-go-lucky barbarian who just wants to magically punch things?  Something else? Working backwards from "what type of character do I want to play?" can be really helpful in deciding your backstory.  Once you know what type of player you want Gorden to be, then you can work backwards from that and figure out how they became that person.  

If you could provide a bit more information of what you are looking for in a backstory/start-of-campaign character, that could be helpful to those trying to create a backstory.  Otherwise, I am sure folks could come up with a million different backstories that all fit the parameters you spelled out--and only a fraction of them might be something you actually want to play.  I would be happy to provide some thoughts if you could give a bit more information on what you want. 

Based on what you have in your original post I'd go for something like along these lines:

Gorden was captured and sold into slavery at a young age. He grew up in slave pits and learned to use his fists to defend himself. He was sold onto a new owner that turned out to be a Giant Soul Sorcerer (thats from an Unearthed Arcana) who 'trained' Gorden by using ritual magics to imbue him with some special abilities. Gorden found fame, fortune and eventually both his freedom and love with the Sorcerers daughter. When the Sorcerer found out that Gorden and his daughter had eloped he had Gorden arrested for 'Kidnapping'. After 3 years in prison Gorden has been released but also exiled and must now find his fame and fortune again, fortunately the Sorcerers rituals are still in place but Gorden must learn how to activate them again.

Goliaths are Giant-kin, so you don't need to reach too far for the runic knowledge, it can be something your character learned from his family.

Quote from Caerwyn_Glyndwr >>

First thing you have to decide is what kind of character do you want Gorden to be at the start of the campaign.  Is he fleeing from something terrible?  Does he have a tragic past he wants to escape?  Is he looking for redemption? Is he a happy-go-lucky barbarian who just wants to magically punch things?  Something else? Working backwards from "what type of character do I want to play?" can be really helpful in deciding your backstory.  Once you know what type of player you want Gorden to be, then you can work backwards from that and figure out how they became that person.  

If you could provide a bit more information of what you are looking for in a backstory/start-of-campaign character, that could be helpful to those trying to create a backstory.  Otherwise, I am sure folks could come up with a million different backstories that all fit the parameters you spelled out--and only a fraction of them might be something you actually want to play.  I would be happy to provide some thoughts if you could give a bit more information on what you want. 

 I think Gordon wants to prove he is worthy of something with greater and greater actions - he is not altitude adapted, and perhaps he was disowned, or ran away after being thought of as insufficient too many times. 

That kind of backstory has three-to-four components, depending on how you want to set up your character:

1. Why your character felt insecure.

2. What occurred during the period he was insecure.

3. What was the catalyst for him deciding to better himself?

4. What has he done since deciding to better himself? This one is optional - perhaps he only just made the choice and the campaign itself is his first effort at betterment. 

First decide if you want him to be recently insecure and new to his desire to prove worth, or if he has been trying to prove his worth for a while before the campaign starts. Those are very different personalities to play and will decide if you need point four (or how much you want to put into point four).  Now that you make that determination, spread the events you listed over the three or four elements you will be using.

Any of your above events could go with any point, and multiple events can be grouped together.  To use being a gladiator as an example for each point to show how versatile everything can be:

1. He was confident as a gladiator until he lost a fight. The officiant spared him, he was booed, and that made him feel insignificant.

2. Being enslaved broke your character’s will, making him feel insignificant - feelings that only got worse as he was forced to kill in the arena. 

3. Joining a ludus (gladiator school) gave him the confidence he needed to want to better himself.

4. Deciding he wanted to make a name for himself, he willingly became a gladiator to seek fame and glory (Roman gladiators were basically sports celebrities - they even appeared on billboards and advertisements, much like today’s endorsement deals).

Same item from your backstory, but it can be tooled to each section.  You could either think about which item belongs in each component, or randomly assign them to a component then figure out how the item fits with that component after assignment.  Either way, by thinking in terms of the character arc’s progression and stages, that should help you come up with a fitting backstory that is unique to you and your character.