What is the standing of Bhagawat Gita on evil forces or devatas?

Bhagawat Gita does not favour devatas. For this reasons, a good section like the Sanyasins, the Advaitins, and Arya Samajees do not recognize devatas. In few Hindu temples, a small section is allotted to worship the devatas apart from the main deity.

But see what Bhagawat Gita says about this in 9:25.

Yanti deva-vrata devan

Pitrn yanti pitr-vratah

Bhutam yanti bhutejya

Yanti mad-yajino ‘pi mam

Those who worship the demigods (devatas) will take birth among the demigods; those who worship the ancestors go to the ancestors; those who worship ghosts and spirits will take birth among such beings and those who worship Me will live with Me.

Again at 7:23 it says that:

Antavat tu phalam tesam

Tad bhvaty alpa-mehasam

Devan deva-yajo yanti

Mad-bhkta yanti mam api

Men of small intelligence worship the demigods, and their fruits are limited and temporary. Those who worship the demigods go the planets of the demigods, but My devotees ultimately reach My supreme planet.