Declare gcamount so it is in scope within the if statement.
//Does the gift certifiacte code refer to a gift certificate with a
if(mymap.contains(o.Gift_Certificate__c)) {
// Declaring gcamount here will make it in-scope within the above if statement.
// A wider scope risks other iterations of the for loop changing the value.
// Set a suitable default value here
decimal gcamount = 0;
//Depending on the payment method, set the gcamount
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) ;
// Error was most likely here in the question
gcu.Amount_Used__c = gcamount;
//add to list
lstgcusToInsert.add(gcu); *****LINE 38 - WHERE THE ERROR EXISTS*****
}