does "get to announce it"= to make known publicly?
“No, gossiping is her default, and you and I both enjoy it from time to time. And occasionally between times. I’m also trying to remember she does have kids, and tends to be overprotective by my gauge. But I don’t have kids.”
“I do, and she’s way over the top. She’d put GPS implants in her kids if she could get away with it. Don’t sit there being tolerant and under-standing. She crossed a line. Everybody, including her best bud, Winnie, knew it. Jesus, Abra, she was gloating about seeing police at Bluff House.”
“I know it. I know it.” Abra pulled up at the cottage with a squeal of brakes. “Most of the gloat was because she got to announce it, but there was plenty left over for Eli’s misery. I’m not tolerant and understand-ing.” She shoved out of the car, snatched her bag, heaved the door shut. “I’m pissed.”
“Good. Me, too. Let’s eat a whole bunch of brownies.”2.what does "most of the gloat"mean?
3. what does "but there was plenty left over for one's misery"mean?

Question

does "get to announce it"= to make known publicly?
“No, gossiping is her default, and you and I both enjoy it from time to time. And occasionally between times. I’m also trying to remember she does have kids, and tends to be overprotective by my gauge. But I don’t have kids.” 
“I do, and she’s way over the top. She’d put GPS implants in her kids if she could get away with it. Don’t sit there being tolerant and under-standing. She crossed a line. Everybody, including her best bud, Winnie, knew it. Jesus, Abra, she was gloating about seeing police at Bluff House.” 
“I know it. I know it.” Abra pulled up at the cottage with a squeal of brakes. “Most of the gloat was because she got to announce it, but there was plenty left over for Eli’s misery. I’m not tolerant and understand-ing.” She shoved out of the car, snatched her bag, heaved the door shut. “I’m pissed.” 
“Good. Me, too. Let’s eat a whole bunch of brownies.”2.what does "most of the gloat"mean?
3. what does "but there was plenty left over for one's misery"mean?

Answer

"To announce" is to make known publicly, yes. "Most of the gloat" refers to the gloating that this woman was doing about seeing police at Bluff House. Gloating is bragging. In the sense of gossiping, gloating is describing how much she enjoys gossiping...that she's proud of being able to gossip about things. "...but there was plenty left over for Eli's misery" refers to, I guess, the gloating. I am assuming that Eli does not like hearing this woman gossip, and is miserable when he has to sit and listen to it.

This excerpt is a little strange. Haha. Hope this helped.