As far as your declaration for gcamount, I think you would be OK putting it right after the for statement. You then need to remove the multiple declarations in the if statements and just have the assignment: `gcamount = o.Amount;`, etc.

Also, in the first if statement, check the logical operators in your code; they should be doubled up - `&&` and `||`.  

Here is my modification on your for loop-  

`for (Opportunity o : trigger.new){

        decimal gcamount;  //consider setting a default value here
				
	    //Is the opportunity in question have a gift certificate as one of the payment methods?
	//remember to double up logical operators
        if ((o.Pay_Method__c == 'Gift Certificate') || (o.Pay_Method__c == 'Multiple Methods' && (o.Multiple_Methods_1__c == 'Gift Certificate' || o.Multiple_Methods_2__c == 'Gift Certificate' )) && o.CreatedById !== '**********' ){

            //Does the gift certifiacte code refer to a gift certificate with a 
            if(mymap.contains(o.Gift_Certificate__c)){

                //Depending on the payment method, set the gcamount
	//double check the logic here to make sure one of these will always be true
	//or add a final else statement to set value, if a default was not set in declaration
                if (o.Pay_Method__c == 'Gift Certificate'){
                    gcamount = o.Amount;
                }
                else if (o.Multiple_Methods_1__c == 'Gift Certificate'){
                    gcamount = o.Multiple_Methods_Amount_1__c ;
                }
                else if (o.Multiple_Methods_2__c == 'Gift Certificate'){
                    gcamount = o.Multiple_Methods_Amount_2__c ; 
                }

                //Create New Gift Card Usage
                Gift_Card_Usage__c gcu = new Gift_Card_Usage__c();
                gcu.Journal_Gift_Card_Usage__c = mymap.get(o.Gift_Certificate__c) ;
                gcu.Amount_Used__c = gcamount;

                //add to list
                lstgcusToInsert.add(gcu); *****LINE 38 - WHERE THE ERROR EXISTS*****
            }   
        }
    }`


Hope this helps