This started as a comment but then really go too long so I am posting an answer:
@yarun, I am also using GnuCash just like you as a non-accountant. But I think it really pays off to get to know more about accounting via GnuCash; it is so useful and you learn a lot about this hundreds of years old double entry system that all accountants know. So start learning about 5 main accounts and debits and credits, imho. It is far easier than one can think.
Now the answer: even without balancing amounts exactly program is very useful as you still can track your monthly outgoings very well. Just make/adjust some reports and save their configurations (so you can re-run quickly when new data comes in) after you have classified your transactions properly. If I still did not know what some transactions were (happens a lot at first import) - I just put them under
Expenses:Unaccounted Expenses - thus you will be able to see how much money went who knows where. If later you learn what those transactions were - you still can move them to the right account and you will be pleased that your reports show less unaccounted money.
How many transactions to import at first - for me half a year or a year is quite enough; once you start tracking regularly you accumulate more date and this becomes a non-issue. Reflecting that personal finance is more about behaviour than maths and that it is more for the future where your overview of money is useful.
Gnucash wil learn from import to import what transactions go where - so you could import say 1 or 3 month intervals to start with instead of a while year. No matter what - I still glance at every transaction on import and still sometimes petrol expense lands in grocery (because of the same seller). But to spot things like that you use reports and if one month is abnormal you can drill down to transactions and learn/correct things. Note that reports are easy to modify and you can save the report configurations with names you can remember. They are saved on the machine you do the accounting - not within the gnucash file. So if you open the file (or mysql database) on another computer you will miss your custom reports. You can transfer them, but it is a bit fiddly. Hence it makes sense to use gnucash on your laptop as that you probably will have around most often.
Once you start entering transactions into GnuCash on the day or the week you incur the expense, you are getting more control and it is perhaps then you would need the balance to match the bank's balance. Then you can adjust the
Equity:Opening Balances to manipulate the starting sums so that current balances match those of your bank. This is easy.
When you have entered transactions proactively (on the day or the week) and then later do an import from bank statement the transactions are matched automatically and then they are said to be reconciled (i.e. your manual entry gets matched by the entry from your statement.)
So for beginning it is something like that. If any questions, feel free to ask. IMHO this is a process rather a one-off thing; I began once - got bored, but started again and now I find it immensely useful.