func (s *GroupWikisService) EditGroupWikiPage(gid interface{}, slug string, opt *EditGroupWikiPageOptions, options ...RequestOptionFunc) (*GroupWiki, *Response, error) {
	group, err := parseID(gid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("groups/%s/wikis/%s", pathEscape(group), url.PathEscape(slug))
	req, err := s.client.NewRequest(http.MethodPut, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	w := new(GroupWiki)
	resp, err := s.client.Do(req, w)
	if err != nil {
		return nil, resp, err
	}
	return w, resp, err
}
EditGroupWikiPage Updates an existing wiki page. At least one parameter is
required to update the wiki page.

GitLab API docs:
https://docs.gitlab.com/ee/api/group_wikis.html#edit-an-existing-wiki-page
package gitlab
import (
	"fmt"
	"net/http"
	"net/url"
)
type GroupWikisService struct {
	client *Client
}
type GroupWiki struct {
	Content string          `json:"content"`
	Format  WikiFormatValue `json:"format"`
	Slug    string          `json:"slug"`
	Title   string          `json:"title"`
}
func (w GroupWiki) String() string {
	return Stringify(w)
}
type ListGroupWikisOptions struct {
	WithContent *bool `url:"with_content,omitempty" json:"with_content,omitempty"`
}
func (s *GroupWikisService) ListGroupWikis(gid interface{}, opt *ListGroupWikisOptions, options ...RequestOptionFunc) ([]*GroupWiki, *Response, error) {
	group, err := parseID(gid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("groups/%s/wikis", pathEscape(group))
	req, err := s.client.NewRequest(http.MethodGet, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var gws []*GroupWiki
	resp, err := s.client.Do(req, &gws)
	if err != nil {
		return nil, resp, err
	}
	return gws, resp, err
}
func (s *GroupWikisService) GetGroupWikiPage(gid interface{}, slug string, options ...RequestOptionFunc) (*GroupWiki, *Response, error) {
	group, err := parseID(gid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("groups/%s/wikis/%s", pathEscape(group), url.PathEscape(slug))
	req, err := s.client.NewRequest(http.MethodGet, u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	gw := new(GroupWiki)
	resp, err := s.client.Do(req, gw)
	if err != nil {
		return nil, resp, err
	}
	return gw, resp, err
}
type CreateGroupWikiPageOptions struct {
	Content *string          `url:"content,omitempty" json:"content,omitempty"`
	Title   *string          `url:"title,omitempty" json:"title,omitempty"`
	Format  *WikiFormatValue `url:"format,omitempty" json:"format,omitempty"`
}
func (s *GroupWikisService) CreateGroupWikiPage(gid interface{}, opt *CreateGroupWikiPageOptions, options ...RequestOptionFunc) (*GroupWiki, *Response, error) {
	group, err := parseID(gid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("groups/%s/wikis", pathEscape(group))
	req, err := s.client.NewRequest(http.MethodPost, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	w := new(GroupWiki)
	resp, err := s.client.Do(req, w)
	if err != nil {
		return nil, resp, err
	}
	return w, resp, err
}
type EditGroupWikiPageOptions struct {
	Content *string          `url:"content,omitempty" json:"content,omitempty"`
	Title   *string          `url:"title,omitempty" json:"title,omitempty"`
	Format  *WikiFormatValue `url:"format,omitempty" json:"format,omitempty"`
}