H
Pretty simple, if a legend like Rayleigh is very aware that he can't beat Teach so he found another way to Win without fighting than Garp is very capable of the same since he allied to Roger to beat Rocks (a crew he knew he wouldn't be able to win by himself).
So if Garp is enganging at Aokiji without trying anything else (don't say shouting him to comeback to marines was really him trying to avoid a fight) is because he knows he is capable of doing it.
And even that Rayleigh had no doubt when going against Kizaru
So Garp is sure that he is capable of handling Aokiji and he surely has the ability to know that just like he was sure he would kill Akainu back at Marineford
Not only that but Aokiji is clearly incapable of knowing what are his limits otherwise he wouldn't have fight Akainu neither would have free Teach's crew
So if Garp is enganging at Aokiji without trying anything else (don't say shouting him to comeback to marines was really him trying to avoid a fight) is because he knows he is capable of doing it.
And even that Rayleigh had no doubt when going against Kizaru
So Garp is sure that he is capable of handling Aokiji and he surely has the ability to know that just like he was sure he would kill Akainu back at Marineford
Not only that but Aokiji is clearly incapable of knowing what are his limits otherwise he wouldn't have fight Akainu neither would have free Teach's crew