Will I still need to fill in a joint sponsor affidavit of support after my F1 student visa expires this October?

No.

No, you can still adjust status even if your visa is expired as long as you can prove a lawful entry. If the income of your US citizen spouse is insufficient, you will need to get a joint sponsor to sign the affidavit of support on your behalf.

As a spouse of a U.S. citizen, you can apply for adjustment of status even though your non-immigrant status had expired. If your family consists only of your husband and yourself, and you husband can show on the affidavit of support that he earns $18,913 a year or more, you do not need a co-sponsor.

Your application will not be denied due to your expired visa, and it sounds as though you should meet the affidavit of support requirements.

Your husband should file the I-130 and you the I-485 soon. Since you entered the country legally, you still can file the I-485 after your F-1 status expires. But I would not wait too long. If there are just the 2 of you, your husband's GI bill income of $2,000 a month should be enough to support the affidavit of support. But Immigration may ask for a co-sponsor if the income is temporary. How long will he receive this income? Is your husband a student too and not working? I would file the papers now and if your husband's affidavit of support is not adequate, you should be given an opportunity to get a co-sponsor. Also, will you be able to get a job soon? You can get EAD after you file the I-485 package. You can add your income to your husband's in the same affidavit. You may be able to do this at the interview.

You are still eligible for adjustment of status if you entered the country legally even if you are now out of status; however, you should file your application as soon as possible to avoid any problems with ICE.