Hi Mei Fong Lee,
Have a look at
Technical support with HMRC online services
regarding your government gateway problem.
If you intend to submit your tax return online, you will need to buy 3rd party commercial software.
You buy the software, fill out their tax return and submit it. You will need your government gateway id and password to do this.
Alternatively, you can send a paper tax return. SA100 (main tax return), SA105 (income from property) and SA109 (residence) please see
Self Assessment tax return forms
Paper tax returns must be in HMRC possession before 31 October, to avoid penalties.
Thank you