The short answer
There is no one-click importer for a whole Mailchimp account. Your audience moves as a CSV export from Mailchimp and a contact import into HighLevel, email designs can be imported from a Mailchimp campaign URL, and forms, landing pages and automations are rebuilt by hand. HighLevel's Mailchimp migration guide follows that order.
Export your audience and import it as contacts
HighLevel's guide says to export audience data from Audience, All contacts in Mailchimp as CSV files. Before importing, create HighLevel custom fields for any Mailchimp merge fields you want to keep, because a CSV column can only map to a field that already exists. Import the file from Contacts, map each column, and tag the batch with the audience name so you can rebuild segments later as Smart Lists. Keep unsubscribed and cleaned addresses out of the list you plan to email; if you import them for records, a DND column marks those contacts Do Not Disturb on every channel.
Bring over email designs from a campaign URL
In Marketing, Emails, Templates, click New, choose Import Email and pick Mailchimp in the Import from list. Paste the campaign URL from Mailchimp, name the template, choose Open in Design Editor and click Create Template. HighLevel turns headers, images and text into editable blocks. Only campaigns have a URL the tool can read, so to import a saved Mailchimp template, create a campaign from it first. Merge tags and conditional content are not translated; swap them for HighLevel custom values or fields and check the unsubscribe link. The tool reads the public campaign page and never asks for your Mailchimp login.
Rebuild forms, landing pages and automations
Signup forms are recreated in HighLevel's form builder under Sites, Forms and embedded on your site with the code it provides. Landing pages are rebuilt under Sites, Funnels from a template or a blank page. For each Mailchimp automation, write down its trigger, delays, emails and audience conditions, then rebuild it under Automation, Workflows; HighLevel's workflow recipes can shorten common sequences. The guide suggests running both tools in parallel for a short time and backing up every export before you cancel. HighLevel's email service costs $0.675 per 1,000 emails, so estimate your monthly send volume before you switch.
More answers in HighLevel questions. For costs, see GoHighLevel pricing or run the cost calculator.