Can I submit taxes for W2 now and stock gains later ?

No, you can't do that. All your income has to be on one tax return. You can't file until you have all the information. Note that the IRS is not going to start accepting or processing 2020 tax returns until February 12. There is no advantage to filing before then.

NO.   All of your 2020 income goes on the same tax return.  You do not file your tax return bit by bit.  The IRS will not begin to accept 2020 returns until February 12, so you have time to get all of your income and documents together to file correctly.