Find the minimum number of operations (concat, add and subtract) for the numbers 1-9 to equal 100
Nesting

Using nested functions is not a bad practice per se, but all depends on the concrete case. Often they are used to enrich the parameters, to share with locally scoped values, or to hide the def from other members of the same object.

In many cases, they are just equivalent of functions with private visibility. But when too much nesting appears, I'd suggest to flatten the things for the sake of readability.

In this example I don't see any special reason to make the function nested. It does not use any of the values available to gchq. Keeping it nested would not be a mistake, but the flat approach seems cleaner:

def gchq2(): Unit = solve("0", 1, concat)

private def solve(total: String, 
                  currentValue: Int, 
                  f: (String, Int) =&gt; String) = ???


Moreover, other referenced functions (plus, minus...) are flat, so all of the pieces are well kept at same level.

Other Considerations

Returned values. The Int return type from gchq() seems suspicious. I hope to understand the meaning (1 for successful result, 0 for no solution found), but the returned values are hugely falsified. solve() will return an Int only from the last call of solve(newtotal, currentValue + 1, plus), the others will remain hidden. Since the solutions are output into the console, I'd suggest to return nothing from solve: 

private def solve(total: String, currentValue: Int, f: (String, Int) =&gt; String): Unit = ???


Conditinals. There are two checks for currentValue == 9. One should be enough:

if (currentValue == 9) {
  if (evaluate(newtotal.split("\\s").toList) == 100) {
    ...
  }
}
else {
  ...
}


It introduces one more case of nested ifs, but is fully justified here: this is the end condition of the calls sequence, while else adds more recursion.

Naming. Names like gchq or f are awful and should be changed to more meaningful.