If you have just started using Simply where did you get your opening balance sheet entries? It sounds as though you did not enter any balances carried forward from the previous year.
You should have taken all the balance sheet figures from your financial statements at the end of your previous fiscal year and entered them as opening balances to start this fiscal year.
It is not to late to do this now, although you will need to do it in a couple of steps and it is a little more complicated now.
First enter all balances in accounts that are not either A/R or A/P through one general journal entry. You will probably have an offsetting amount that will be posted either as a dr or a cr to Retained Earnings. Date this entry on the first day of your fiscal year.
Next enter all A/R accounts (setting up your customers as you go along) through individual entries through the Sales Journal. Again you will need to offset these entries to Retained Earnings. Again these entries will all be dated as of the first day of your fiscal year.
Next do the same of all A/P accounts, this time through the Purchases Journal. You will again be offestting these entries to Retained Earnings and dating them the first day of your fiscal year.
This should give you an opening balance sheet which matches your balance sheet from your previous year. If it does not, it is probably because you have made current year entries on the first day of your current fiscal year. Run a G/L report for the first day of your fiscal year and check.
Once you are confident that your opening balances are correct you should be able to post an entry to record the receipt of a GST refund for the previous year. Your entry will be a dr to the bank, a dr to GST charged on sales and a cr to GST paid on purchases. Your GST owing (refund) account should be a sub-total account so you cannot post to that account. You can only post to actually accounts, not total or sub-total accounts.
In future when you prepare your GST returns - take a GL report for the appropriate period which includes all GST accounts from your balance sheet and all revenue accounts from your income statement. This is the only accurate way to get the right numbers. I recommend that everybody set up the CRA as both a customer and a vendor. That way when you prepare your GST return you can easily record either a receivable or a payable as appropriate.