Hi so I have a friend trying to make a homebrew subclass for bard called College of the Gambler. The problem is that the subclass has a level 10 ability that replaces magical secrets. How can they override that for the subclass to actually work?

Unfortunately, at this point in time, you cannot use homebrew to override class abilities right now on Beyond. I do not see Beyond working on injecting more flexibility into their homebrew system in the near future due to a long list of other priorities.

The best you can do is to homebrew a feat (or a collection of feats, depending on how complex the subclass ability is) and have the bard take that feat at level 10 and leave Magical Secrets blank and pretend it is not there.

So how can I make "Go For Broke

Beginning at 10th level, you've learned to truly push your luck in the face of danger.

When you or an ally spend one of your bardic inspiration dice, you may choose to re-roll the dice, adding the results together. You may continue to re-roll this bardic inspiration dice as many times as you want in this way, however, if the total of your rolls would exceed the maximum value of your bardic inspiration dice, the dice instead gains no benefit, and the creature using the dice gains a penalty to the triggering ability check, attack roll, or saving throw equal to the difference." This into a balanced feat

Quote from MagisterHelix >>

So how can I make "Go For Broke

Beginning at 10th level, you've learned to truly push your luck in the face of danger.

 

When you or an ally spend one of your bardic inspiration dice, you may choose to re-roll the dice, adding the results together. You may continue to re-roll this bardic inspiration dice as many times as you want in this way, however, if the total of your rolls would exceed the maximum value of your bardic inspiration dice, the dice instead gains no benefit, and the creature using the dice gains a penalty to the triggering ability check, attack roll, or saving throw equal to the difference." This into a balanced feat

I think you are better off visiting the homebrew section of the forums if you want more homebrew specific help, but I will offer what little insight I have.

In general, you only need to mess with the homebrew tools on a "deeper" level if the ability requires Beyond to display or calculate something. A lot of abilities can be just left as text, even if it involves calculations, as there are limitations to Beyond's ability to calculate things. For example, even though Heavy Armor Master involves calculation since it reduces certain damage by 3, it is not something Beyond can calculate, and it is something the user have to calculate manually. In this case, while Beyond can handle the addition of a variable number of dice rolls from a static formula (Xd12+Yd10=Z), Beyond canNOT handle the addition of what is essentially a variable number of formulas (Xd12+Yd10=Z; Z+Xd12+Yd10=A; A+Xd12+Yd10=B; B+Xd12+Yd10=C; and so on). Since the player can decide whether continue or stop adding dice rolls, the player has to track the sum of the dice rolls manually.

Beyond does not really support temporary effects, so things like applying a one time penalty to your dice rolls have to be done manually. And for one time effects, this is honestly better off left as text only.

In short, I think your subclass ability is fine to be left as just text.

If you are wondering whether your subclass ability is balanced, I think it is fine, but I am not the best judge of what is balanced or not.