Compute predict output according to tree models

How to compute predict output according to the tree models exported before?
In regression, the output isn’t equal to the sum of every tree.
Like this,
params = {
‘booster’: ‘gbtree’,
‘objective’: ‘reg:gamma’, # 回归的损失函数,gmma回归
‘gamma’: 0.1,
‘max_depth’: 5,
‘lambda’: 2,
‘subsample’: 0.7,
‘colsample_bytree’: 0.7,
‘min_child_weight’: 3,
‘silent’: 1,
‘eta’: 0.1,
‘seed’: 1000,
‘nthread’: 4,
}
plst = params.items()

and the tree models are as following:
booster[0]:
0:leaf=0.0977716669
booster[1]:
0:leaf=0.0975570381
booster[2]:
0:leaf=0.0972924531
booster[3]:
0:leaf=0.0970089585
and the sum is about 0.4 but the prediction is about 0.7?
should I use eta when compute the sum?

No, please see XGBoost learning-to-rank model to predictions core function?