Natsu has not seen Gildarts since before the GMG I think whatever they talk about would also include the GMG and Tartarus, probably what happen, his failures and regrets maybe even what F Rogue said I'm Lucy did come up maybe either F Lucy or how he left Lucy and miss her though I don't think a love confession. The opening up of all that or maybe some of it is why he is blushing,shaking and slightly sweating like when Asuka asked if he kisses Lucy. Though why do you think he pointed to the sky?
I’m confident that Natsu told Gildarts about Igneel and the events of Tartaros. He might’ve been pointing at the sky to indicate Igneel and Acnologia’s battle, or maybe even his perspective as he saw Igneel fall. :(
I think Natsu’s shaking, blushing, and sweating was probably him talking about his own reaction to Igneel’s death, and his desire to grow strong enough to protect his friends. He could’ve talked about his feelings of helplessness and sadness, and his determination that he would never let Acnologia—or anyone—take away someone else he cared about.
Tbh, I don’t think Natsu brought up the Asuka incident at all. But I like that he might’ve also brought up what happened with f!Lucy in the GMG. It’s something we never saw Natsu reflect upon afterward, so it’s nice to be able to imagine he got it off his chest here.
I think Gildarts’ slight, proud smile at the end of the series of panels was him responding to Natsu’s conversation as a *whole*. He was able to see how Natsu had matured since their last meeting.
Doing this as a textless chapter was a brilliant move by Mashima. Now we can each come up with our own interpretations of what was said, and leave the chapter with the happy thought that Natsu addressed all the issues he ought to have. ;)