Under" Guild Expeditions" in the wiki there is useful information.
Guild Expeditions - Forge of Empires - Wiki EN
Specifically the link under heading of Expedition Difficulties:
Cost of unlocking difficulties Costs of Unlocking Difficulties - Forge of Empires - Wiki EN
I have enclosed the links because I forget where to find them. The last link allows you to calculate the cost of you playing GE. Remember it is calculated at status at start of game.
Also to adjust statement listed in another post: There are three GBs that provide goods directly to the Guild along with other boosts, namely Observatory (no age), Atomium (modern age), The ARC (future age).
Your will want one or all of these. They aid your Guild's overall status and should gain all guild members benefits as seen of the Guild level.
Kindly note they provide CURRENT ERA goods so it shouldn't be necessary for the player to add extra goods for GE levels 3 and 4. Level 2 requires previous era goods, which are perhaps covered by another guildmates current goods. Players would need to level up the GBs to maintain the needed goods. And if asked to donate for GE it is probably your previous era goods that are needed.
A problem arises when there is a large difference in Era Levels and no one is producing the needed goods.
When reading the wikis note that it is only Founders and Leaders who can pay from the Treasury to unlock the next GE level. The red numbers indicate that they have not yet paid, possibly due to not having the goods (know your Guild). Our donating them will allow payment but, they need to act to get it done.
It would be wise to assign many Leaders (ones who finish Lvl 1 first) so game momentum isn't lost by not opening the level soon enough. Each member should be able to complete level 1 (free). Doing so could be a Guild requirement and would help the guild to level up it's benefits.