Yes we can configure the GP to present the login banner, to do that go to tab Network > Global Protect > Click on Portals under Portal configuration we can assign the default page in the field Custom Login Page
You can also modify the GP welcome page , this can be done by accessing the Response page under device tab:
Device> Response page > GlobalProtectPortal Login Page
Export it, edit it and import it back and commit.
Also please refer below documents for more information to modify the GP portal Login Response page:
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