His galaxy impact wasn't meaningfully distinguished from Coby's Honesty Impact and Coby hasn't been suggested to be a Haoshoku user.
The main reason to believe Garp has Haoshoku is because of Dragon and Luffy, and a Top Tier Haoshoku user should have Haoshoku coating.
Unless you count the biased Sanji bros, which believe he's > Yc1, or even low top tier