Dapper helper service
Naming

I recommend not using acronyms or shortened names where possible for your variables. You might as well use the full name, it aids in quick reading.

e.g.

public dynamic Insert&lt;T&gt;(T dataTransferObject) where T : class
{
    return Execute((x, t) =&gt; x.Insert(dataTransferObject,t));
}


Although for that method item may work just as well.

The same goes for single-letter variable names in your lambda expressions. If there's only parameter to your lambda, it's probably ok, but with multiple parameters it can quickly become confusing.

return Execute((connection, transaction) =&gt; connection.Insert(dataTransferObject,transaction));


Shows the exact purpose of your lambda much more clearly.

I also note that you have a method called GetList that does not return a List. This is just plain confusing.

Lastly, you should avoid putting the type of the variable in its name. 

E.g.

public void Insert&lt;T&gt;(List&lt;T&gt; dtoList)


would be better as:

public void Insert&lt;T&gt;(List&lt;T&gt; dtos)


Otherwise it can lead to confusion should you decide to change it to a different type, and forget to update the variable name.

var

You have used var appropriately and consistently, making this (as far as I'm aware) the first code review I've ever posted that does not contain some guidance on var. Well done.

Parameter types

You take a List with this method:

public void Insert&lt;T&gt;(List&lt;T&gt; dtoList) where T : class
{
    IEnumerable&lt;T&gt; enu = dtoList;
    // yadda yadda yadda...


And then immediately convert it to an IEnumerable in your body. Just take the parameter as IEnumerable in the first place, passing a List in will still work. By requiring a List you're actually making it harder to use your own function (because you can't, for example, pass in an array) for no reason at all.