I love your kid’s names so far! For the sibset, I would recommend:
[name]Griffin[/name]
[name]Grant[/name]
Godric (sorry, I’m a [name]True[/name] Blood fan)
[name]Graham[/name]
[name]Grayson[/name]
[name]Grady[/name] = meh I prefer [name]Brady[/name]
[name]Glenn[/name] is my father’s name. I feel it looks nicer with two N’s, but maybe just cuz I am used to it. [name]Glenn[/name] is good as a mn. It is quite dated and seems more like a dad name to me… and the only famous [name]Glenn[/name] is [name]Glenn[/name] Close (a woman!).
I adore [name]Gus[/name]. It could be a nn for [name]Angus[/name] or [name]August[/name]/[name]Augustus[/name]/[name]Augustin[/name].
What about:
[name]Gwynn[/name] (love this!)
[name]Guy[/name]
[name]Gawain[/name]
[name]Gulliver[/name]
[name]Galloway[/name]
[name]Gervais[/name]
[name]Gideon[/name]
[name]Gilbert[/name]
Gilderoy
Gilligan
[name]Gilmore[/name]
[name]Gus[/name]
Goodwin
[name]Greer[/name]
[name]Gregor[/name]
[name]Griffin[/name]
[name]Gustav[/name]
[name]Hope[/name] this helps 
Just for the record, here are just some of the other famous/successful Glens that I found online. There were too many to list, but here is a sampling:
Musicians:
[name]Glen[/name] [name]Campbell[/name] (b. 1936) [name]Jazz[/name] band leader - appeared in some Century [name]Fox[/name] movies
[name]Glenn[/name] [name]Miller[/name] (b. 1904) Country singer
[name]Glen[/name] [name]Gray[/name] (b. 1900) successful career 1922-1950 ([name]Orange[/name] [name]Blossom[/name]/Casa [name]Loma[/name] Orchestra)
[name]Glen[/name] [name]Stetson[/name] (b. 1940) The Diamonds, 1950s/60s
[name]Glen[/name] [name]Buxton[/name] (b. 1947) Lead guitarist, songwriter
[name]Glen[/name] Matlock (b. 1956) - Punk rocker
Athletes:
[name]Glen[/name] D. [name]Russell[/name] (b. 1915) - [name]Major[/name] League Baseball: [name]Boston[/name] [name]Red[/name] Sox and Chicago Cubs
[name]Glen[/name] D. Flanigan (b. 1926) - Professional Boxer
[name]Glen[/name] Holloway (b. 1948) - Professional Football: Chicago Bears and [name]Cleveland[/name] Browns
[name]Glen[/name] Gondrezick (b. 1955) - Professional Basketball player and broadcaster
[name]Glen[/name] [name]Keeley[/name] (b. 1970) - Professional Rodeo
[name]Glen[/name] Combs (b. 1946) - Basketball
[name]Glen[/name] [name]Harmon[/name] (b. 1921) - Professional Hockey
[name]Glen[/name] Gorbous (b. 1930) - [name]Major[/name] League Baseball: In the Guiness Book of Records for his throwing distance
[name]Glenn[/name] W. Davis (b. 1924) - Football, won Heisman Trophy in 1946, U.S. Military Academy at West Point team
[name]Glen[/name] Osbourne (b. 1971) - Professional Wrestler
Actors:
[name]Glenn[/name] Ford (b. 1916)
[name]Glen[/name] Hambly (b. 1963)
[name]Glen[/name] C. [name]Gordon[/name] (b. 1915)
[name]Glen[/name] Cavender (b. 1883)
[name]Glenn[/name] Beck (b. 1964) Talk Show Host / Journalist
[name]Glenn[/name] Ordway (b. 1951) - sports radio and television personality
Businessmen:
[name]Glen[/name] W. [name]Bell[/name] - founder of Taco [name]Bell[/name]
I really love [name]Grady[/name]!
WoW! I’m impressed by your list of famous, successful [name]Glen[/name]'s!!!
Thanks for your research…I will never look at [name]Glen[/name]/[name]Glenn[/name] the same way again 
Also, Thanks to All for helping me narrow down my “G” name…I was recently really bummed when someone commented that the name [name]Grady[/name] reminded her of the word ‘degrading’…I thought that was a stretch…but, no mom-to-be wants her baby ‘degraded’.
But thanks to everyone who posted comments, I feel like there are some really nice choices to consider my little guy!!! 
I think [name]Graham[/name] would go great with [name]Gavin[/name].
[name]Gareth[/name] is my favorite “G” name, it was on our list for quite a while. [name]Just[/name] thought I’d add my two cents! Good [name]Luck[/name], you have some great choices here! I love your children’s names! I know a [name]Glen[/name] who is in the [name]Air[/name] Force.
[name]Grady[/name] feels akin to [name]Brody[/name] in cutesy trendiness. Rather nickname-ish, especially for a boy.
I like [name]Gregory[/name]/[name]Greg[/name] but don’t see it as being usable, yet. [name]Gordon[/name] is going to be awhile longer, too.
I like [name]Gryphon[/name]/[name]Griffin[/name] and [name]Grayson[/name] quite a bit, along with the more classic [name]George[/name].