The biggest reason why people consider Gol D. Roger and Monkey D. Garp to be close in strength was that Roger said that they nearly killed each other multiple times.
However, that may have been when they were younger or when Roger was at a disadvantage.
Here are some things suggesting that Roger and Whitebeard may have been on another level than Garp when they were in their primes.
1) Buggy in chapter 233 stated that Whitebeard was the only man capable of going neck to neck with the former pirate king Roger.
2) Whitebeard was considered the world’s strongest man, and not Garp. If Whitebeard was equal to Roger, that would mean that Roger was also noticeably stronger than Garp. Even after being weakened by sickness, Whitebeard was recognized as the strongest.
3) Shiki, even though he admitted that Roger’s power was enormous even though he hated him, and even respected Whitebeard, called Sengoku and Garp “weak scum”.
4) Whitebeard was considered the ruler of the seas in his time without Roger. It would not have been the case if Garp was equal or even close to Roger/Whitebeard because the marines wouldn’t allow a pirate to hold such a title. Garp himself called Whitebeard the ruler of the seas.
And Doflamingo too
All these things lead me to conclude that Garp wasn’t actually close to Whitebeard or Roger in their prime. Maybe he was able to damage Roger when they were younger, but not later on.
So what do you think?
*Theory by Ichijinijisanji