It uses the specified GVK, but then ignores it

git_repository.yaml

---
apiVersion: source.toolkit.fluxcd.io/v1beta2
kind: GitRepository
metadata:
  name: hello-world
  namespace: hello-world
spec:
  interval: 10m0s
  ref:
    branch: main
  url: https://github.com/yitsushi/hello-world

main.go

package main

import (
	"context"
	"log"

	sourcev1 "github.com/fluxcd/source-controller/api/v1"
	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
	"k8s.io/apimachinery/pkg/runtime"
	"k8s.io/apimachinery/pkg/runtime/schema"
	"sigs.k8s.io/controller-runtime/pkg/client"
	"sigs.k8s.io/controller-runtime/pkg/client/config"
)

func main() {
	c, err := client.New(config.GetConfigOrDie(), client.Options{})
	if err != nil {
		log.Printf(" !! error: %s\n", err)

		return
	}

	key := client.ObjectKey{Namespace: "hello-world", Name: "hello-world"}

	log.Printf(" ++ gitrepo v1\n")
	resp := getResource(c, key, schema.GroupVersionKind{
		Group:   "source.toolkit.fluxcd.io",
		Kind:    "GitRepository",
		Version: "v1",
	})

	logGitRepository(resp)

	log.Printf("\n\n ++ gitrepo v1beta2\n")
	resp = getResource(c, key, schema.GroupVersionKind{
		Group:   "source.toolkit.fluxcd.io",
		Kind:    "GitRepository",
		Version: "v1beta2",
	})

	logGitRepository(resp)

	log.Printf("\n\n ++ gitrepo v99omega18\n")
	resp = getResource(c, key, schema.GroupVersionKind{
		Group:   "source.toolkit.fluxcd.io",
		Kind:    "GitRepository",
		Version: "v99omega18",
	})

	logGitRepository(resp)
}

func logGitRepository(obj *unstructured.Unstructured) {
	var repo sourcev1.GitRepository

	err := runtime.DefaultUnstructuredConverter.FromUnstructured(obj.UnstructuredContent(), &repo)
	if err != nil {
		log.Printf(" !! error: %s\n", err)
	}

	log.Printf(" -- unstructured gvk: %+v\n", obj.GroupVersionKind())
	log.Printf(" --         repo gvk: %+v\n", repo.GroupVersionKind())
	log.Printf(" --         repo uid: %v\n", repo.GetUID())
	//log.Printf(" --         repo gvk: %+v\n", repo)
}

func getResource(c client.Client, objKey client.ObjectKey, gvk schema.GroupVersionKind) *unstructured.Unstructured {
	u := &unstructured.Unstructured{}
	u.SetGroupVersionKind(gvk)

	err := c.Get(context.Background(), objKey, u)
	if err != nil {
		log.Printf(" !! error: %s\n", err)
	}

	return u
}

output

2023/07/04 10:16:49  ++ gitrepo v1
2023/07/04 10:16:49  -- unstructured gvk: source.toolkit.fluxcd.io/v1, Kind=GitRepository
2023/07/04 10:16:49  --         repo gvk: source.toolkit.fluxcd.io/v1, Kind=GitRepository
2023/07/04 10:16:49  --         repo uid: 019dc27b-3bc2-4fb4-aa59-971411b4f2ab
2023/07/04 10:16:49

 ++ gitrepo v1beta2
2023/07/04 10:16:49  -- unstructured gvk: source.toolkit.fluxcd.io/v1beta2, Kind=GitRepository
2023/07/04 10:16:49  --         repo gvk: source.toolkit.fluxcd.io/v1beta2, Kind=GitRepository
2023/07/04 10:16:49  --         repo uid: 019dc27b-3bc2-4fb4-aa59-971411b4f2ab
2023/07/04 10:16:49

 ++ gitrepo v99omega18
2023/07/04 10:16:49  !! error: failed to get API group resources: unable to retrieve the complete list of server APIs: source.toolkit.fluxcd.io/v99omega18: the server could not find the requested resource
2023/07/04 10:16:49  -- unstructured gvk: source.toolkit.fluxcd.io/v99omega18, Kind=GitRepository
2023/07/04 10:16:49  --         repo gvk: source.toolkit.fluxcd.io/v99omega18, Kind=GitRepository
2023/07/04 10:16:49  --         repo uid:

https://github.com/kubernetes-sigs/controller-runtime/blob/main/pkg/client/client.go#L385-L388